I have a 2002 A4 Avant 1.8T 130K

The stereo speakers started intermittently going in-and out 1 yr ago. The stereo itself stays on and CD keep splaying but sounds goes in and out. It does not fade in and our--speakers are either working or not.
Problem has gotten gradually worse--today if I drive in the city for an hour the sound will be on for ~10min or less.

-this happens to all speakers at once (not just front, back, left or right)
-speakers can go in and out both at standstill and while driving
-bumps possibly cause speakers to go in and out?
-seems to work better in cold weather

What I've done to diagnose:
-pulled amp out shaken it and put it back in -- no effect
-tapped on amp with soft mallet while stereo is on -- no effect
-jostled all wires to and form amp -- no effect
-have not gotten amp checked. Car Toys wanted to charge me $120

someone told me it might be a faulty ground wire?

how do I further diagnose this problem?
trouble shoot ground wire, verify the amp is working?

any help would be most apprecaited

Mike
Denver, CO