whacked electrical/stereo trouble

[Hmonkey]

OK
Ive got a 90 Ford E350. I installed an aftermarket CD player in the van using all new wiring. Heres the trouble, The stereo works great while the vehicle is in the Acc position and when the engine is running in park. Its when I put the van in drive or reverse that I start to have problems. The initial switch from park into a gear will often cause the stereo to go off and then back on (like its losing power). Then when the van is in motion the switching on and off of the stereo seems to coincide with breaking or general decelleration.
I tried all the obvious (to me) things, all the wiring is good with a good solid ground, I checked for a wire or anything which may be coming into contact with the brake pedal itself , new fuses. I even let the van idle in park with the stereo on and had some friends help me shake the van and I was unable to reproduce the results that happen when the van is in gear.....Im stumped.