bazzer: It doesn't sound like the same thing. For a start, I don't think the same symptoms are occurring across different brands (other than two specific Zinwell and JCMatthew boxes which are basically identical, I think).
When I say unresponsive, it's simply that it takes somewhere between the normal time and a really long time for the unit to react to the remote. Eventually it will catch up and perform all the operations. Sometimes it's worse, sometimes it's better and it can be fixed by pulling the aerial, all without a reboot. I don't think it's the firmware "completely crashing", as you put it, I think it's just not able to "keep up" with the current transport stream. Not sure if this matches your experience or not?
Okay, thanks for clarifying. My device locks solid and won't respond to anything at all until switched off at the wall, so you're probably right in saying it's a different problem.