A faulty transistor or IC that has failed, it probably overheats before any other device. You should shut the amp down when it happens, then feel carefully around for the hottest part. I would replace that. Or you can power the amp back up wait till it acts up, then spray that hot part with Servisol Freezer spray. If it comes back on, then you know it's faulty. If not try another part, with the spray, only one at a time.