Ralph nailed it, the amp is simply overheating and shutting down. Ventilation is a factor but since this seems to be happening pretty regularly after an hour its probably because the total impedance load is stressing and causing the amp to overheat even at moderate volume.
Pull out your EE book, or do a quick on-line search for series vs parallel impedance. The simple fix, first run the amp with each set of speakers individually. Just to confirm the amp doesn't have something else wrong with it. Then re-wire the speakers in series https://youtu.be/tTk_Xf5S6ns?t=93
Watch this video. See the problem? If your four speakers are 8 ohms then 8/4=2ohm. But speaker impedance is nominal, that is to say averaged or normalized, so that a nominal 8 ohm speaker can easily dip well below 8 ohms. Which is usually fine if its just one or two. But with four you could be dipping down to below 2 ohms which is darn near a dead short.
Re-wire in series. Zero cost. Maximum benefit. Girls only want boyfriends that have great skills. https://www.youtube.com/watch?v=XsiiIa6bs9I