I have had two periods where this happened with my AVR300. On both occasions over a week the AVR switched off for no obvious reason, and restarted normally when I switched off and on again. After the first period the amp worked without any problems for two years. The last period was about 3 weeks ago, and for the last two weeks it has been fine.
The only thing that I have noticed is that it seemed to only happen when it was using the audio signal from a Humax FreeSat box and when the room was warmer than usual. Still, the base of the unit was cold, suggesting that the cooling fan was working, and the top was warm, but not unusually so. Certainly a cut-out of some sort - possibly being over sensitive for some reason. The amplifier was not on for long periods or overworked.
I hope yours starts behaving again as mine did. Let me know how you get on.