mlab

Pioneer DEH-80PRS cutting out under vibration/high volume

I've done a lot of newbie testing , so this could be quick to diagnose.

Pioneer DEH-80PRS head unit starts cutting out or skipping continually and only stops if I keep the volume pretty low. Any attempt to raise volume at med-high levels makes the sound cut out repeatedly again.

Happens with radio as well (not CD skipping), only at higher volumes.
Happens with engine on or off, only at lower volumes with engine on (higher voltage read on screen).
Not running an amp, just HU and 4 speakers.
Does not happen with other Pioneer head unit used for testing, DEH-X9500BHS. So nothing wrong with wiring, speakers, etc. It's the HU.

HU plugged in and sitting on passengers seat, starts cutting out when I tap on the top cover with my fingers, near the heat sink or antenna, mostly. So, I know road vibration is what starts it.

Lower the volume, stops cutting out, raise the volume, cuts out again. Keeps doing this forever.

Tap on it again a few times and it fixes itself. I can raise volume to max levels again.

Tap again, starts skipping / cutting out again. Tap some more, fixes itself, and so on...

None of this happens with other head unit (DEH-X9500BHS).

Very willing to open the thing up and solder or replace parts. Just need to know what to look for.

Please help!