The amp would be my guess as well, but usually something like what you describe happens after a while of listening, and isn't really fixed that easily.
For example, on my trip back to the parents' house, 2 hours and 20 minutes, I'll listen to my empeg at about 0dB all the way. around the 2 hour mark, sometimes the sound will cut out completely. This is because the amp has overheated and shut off. All I need to do is turn off the empeg and wait about 5 minutes for it to cool down. I also turn on the lower fans of the A/C
You probably have a different problem, but it could be related.