Spontaneous shutdown and HDMI control

This is another one of those questions that has probably been asked and answered several times, but my feeble search skills haven't found any of them.

I have a Denon AVR-1612 hooked up to a Sony KDL-32LL150 via HDMI. I figured out how to get the receiver to turn on when I turn the TV on, and turn off when I turn the TV off. Life was good. But soon (within hours) the two units starting turning themselves off at unpredictable intervals. The intervals are measured in hours rather than days or minutes. When this happens, they both shut down at once. Prior to be introduced to the new receiver, neither the TV nor any other electrical stuff in the room had ever done this before.

Any clues about what to look for would be appreciated.

I forgot to mention that I have already double-checked the sleep timer settings on both components.

wajo's suggestion is exactly on target. Thanks! Although I had checked the sleep timer feature on both units, I had not checked a Sony TV feature called "Idle TV Standby". Turned that off and life is good again.