The Scripture references that Grudem refers to need some clarification. Though I have not taken the time to exegete these passges I believe the following commentaries offer reasonable explanations and defeat Grudem’s analysis.

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Grudem—

Acts 21:4: In Acts 21:4, we read of the disciples at Tyre: “Through the Spirit they told Paul not to go on to Jerusalem.” This seems to be a reference to prophecy directed towards Paul, but Paul disobeyed it! He never would have done this if this prophecy contained God’s very words and had authority equal to Scripture.
Luke relates that the Christians in Tyre seek to dissuade Paul from going to Jerusalem. They have received a revelation from the Holy Spirit (compare I Cor. 14:32) that Paul is going to meet adversities there. This revelation supports Paul’s comment that the Holy Spirit warned him about future imprisonment and hardship (20:23).

Is there a contradiction between the revelations Paul received from the Holy Spirit and those which the believers in Tyre obtained? No, not at all. The Christians in Tyre heard the Holy Spirit say that Paul would meet adversities, but they did not understand the purpose of Paul’s future suffering. Conversely, Paul understood the warnings as confirmation that “he must suffer for [the Lord’s] name” (9:16). He considered these divine revelations to be symbols of God’s grace designed to prepare him for the immediate future. (New Testament Commentary).

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Acts 21:10–11: Then in Acts 21:10–11, Agabus prophesied that the Jews at Jerusalem would bind Paul and “deliver him into the hands of the Gentiles,” a prediction that was nearly correct but not quite: the Romans, not the Jews, bound Paul (v. 33; also 22:29), and the Jews, rather than delivering him voluntarily, tried to kill him and he had to be rescued by force (21:32). The prediction was not far off, but it had inaccuracies in detail that would have called into question the validity of any Old Testament prophet. On the other hand, this text could be perfectly well explained by supposing that Agabus had had a vision of Paul as a prisoner of the Romans in Jerusalem, surrounded by an angry mob of Jews. His own interpretation of such a “vision” or “revelation” from the Holy Spirit would be that the Jews had bound Paul and handed him over to the Romans, and that is what Agabus would (somewhat erroneously) prophesy. This is exactly the kind of fallible prophecy that would fit the definition of New Testament congregational prophecy proposed above—reporting in one’s own words something that God has spontaneously brought to mind.
The IVP New Testament commentary series states, in the light of Acts 19:21, it seems that Paul’s intentions to go to Jerusalem and then Rome were purposes directed by the Holy Spirit and that the warnings were to prepare him. ….. In a symbolic act much like the acted-out prophecies of the Old Testament prophets, Agabus predicted Paul’s coming arrest in Jerusalem. He took Paul’s girdle, the long cloth that was wound several times around his waist, and bound with it his hands and feet. Then, just like an Old Testament prophet, he gave the interpretation of the act, introduced by the usual, “Thus says the Lord,” here expressed in terms of revelation through the Holy Spirit. Paul would be bound by the Jews of Jerusalem and handed over to the Gentiles. The parallel to the fate of Jesus could hardly be more explicit (cf. Matt 20:18f.; Luke 18:32). This was not so much a warning on Agabus’s part as a prediction. Unlike the Christians of Tyre, he did not urge Paul not to go. Rather, he told him what was in store for him. …. The act itself set into motion the event it foretold. It established the reality of the event, the certainty that it would occur. Agabus’s act prepared Paul for the events to come and assured him of God’s presence in those events.

As far as the Roman administration was concerned, Caesarea was the capital of the province of Judea; however, Caesarea was a Gentile city, it was not considered by the Jews as a part of their country in the popular sense of the word. ….

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1 Thessalonians 5:19–21: Paul tells the Thessalonians, “do not despise prophesying, but test everything; hold fast what is good” (1 Thess. 5:20–21). If the Thessalonians had thought that prophecy equaled God’s Word in authority, he would never have had to tell the Thessalonians not to despise it—they “received” and “accepted” God’s Word “with joy from the Holy Spirit” (1 Thess. 1:6; 2:13; cf. 4:15). But when Paul tells them to “test everything” it must include at least the prophecies he mentioned in the previous phrase. He implies that prophecies contain some things that are good and some things that are not good when he encourages them to “hold fast what is good.” This is something that could never have been said of the words of an Old Testament prophet, or the authoritative teachings of a New Testament apostle.

1 Corinthians 14:29–38: More extensive evidence on New Testament prophecy is found in 1 Corinthians 14. When Paul says, “Let two or three prophets speak, and let the others weigh what is said” (1 Cor. 14:29), he suggests that they should listen carefully and sift the good from the bad, accepting some and rejecting the rest (for this is the implication of the Greek word “..” (G1359) here translated “weigh what is said”). We cannot imagine that an Old Testament prophet like Isaiah would have said, “Listen to what I say and weigh what is said—sort the good from the bad, what you accept from what you should not accept”! If prophecy had absolute divine authority, it would be sin to do this. But here Paul commands that it be done, suggesting that New Testament prophecy did not have the authority of God’s very words.
Grudem fails to realize that we also need to judge Scripture (Matt 4) and tell whether it is being used in context, et. al. (Jeremiah 23:36). His whole argument is flawed.

There are so many other problems here, but there is just not enough time …

Reformed and Always Reforming,