Speakers cutting in and out

I could use some stereo advice -- I've got some speakers that cut in and out, usually when I go over a bump. Both the driver's side front and passenger side rear. So much for my fancy bose system ... doesn't sound so great with 4 speakers out.

Audi has replaced the driver's door bass speaker, replaced the head unit twice, added some felt to the back of the speakers to prevent them from shorting, done some silly connector cleaning trick 4 times, all for the front driver's side. Nothing helped for very long, luckily all under warranty :-)

Anybody have any thoughts on what the problem might be? Wiring? Amp?
I've pulled the driver's door panel, and the speaker connections are all solid there. Also the connector on the amp in the back is solid (no crackling/popping at all when I wiggle the connectors around).
I'm thinking the next thing to check is the connector on the back of the head unit. After that I'm out of ideas ... are there any other connectors I can check?