Chevy & GMC Vans

whenever I go down the road my stereo keeps going in and out. it never loses 100% power (display and buttons still work), but it loses enough that the internal amp goes in and out. I thought it was the head unit (aftermarket kenwood), so I replaced it with a clarion I had lying around. I still have the same damn problem. when the van gets jostled (like on a bumpy road or under acceleration/ braking, while turning , or pretty much any movement) dthe radio loses partial power. I have checked the wiring harness and all connections are good. I'm thinking its whatever gives the radio power in the ignition switch (not sure of proper term). everything else seems to be working ok though. also, of the past week, its taking the radio about 5 seconds to get power (display doesnt light up until about 5 seconds after turning on van). before it would turn on instantly. any ideas of what could be causing this issue?
I also had the steering column worked on shortly before the problem started (van wouldnt go into park or turn off) and am wondering if something could have been bumped loose.

did you ever get this figured out? I had same problem, except I had an ooold kenwood tape deck that worked fine with the same wiring setup. Put in the new kendwood and the sound cut in and out even though the disc would keep playing. After putting in a better ground wire and that not working, I cut the original power lead (orange) coming out of the fuse box and ran a 14ga wire from the fuse box to the radio harness behind the dash. radio is good but now my fuel gauge is out, so I guess I traded one problem for another.