I've done all that, except the different display and the shorter cable, because it's not practical.

Not sure if this changes much in terms of pointing the finger at a specific device. Last night the problem was happening and to fix it, I just turned off the amp for 30 seconds and back on again. Ran fine for about 3 hours after that. Does that indicate an AMP issue?

Many amps have had well known issues with the HDMI boards cooking up and dying. Sounds like an ideal candidate for the problem given the symptoms.

I'd certainly try the second HDMI output.

Video the issue if it happens again, then send it in for a service.

The Aventage range have a 4 year warranty, so it should be fine.