Stereo only works with engine not running

[budman k20]

I spent a good part of my weekend wiring up my stereo and sub feeling pretty good thinking I'm done wiring up my truck so now I can finally enjoy tunes...Wrong!! I was so bummed on my maiden run when the radio started cutting in and out like someone was turning the volume up and down every second or two. Then the engine stalled as I was leaving and what do you know... my stereo played normal! So with the ignition switch on but the engine NOT running or with the key in the accessory position my radio plays fine only when the engine is running do I have the problem! I have a good ground to the radio and I'm using the radio feed from the fuse block to power it up. The truck has all new wiring in the cab and I have a HEI ignition. My thoughts are I need some sort of filter but I have never had a problem like this before. Anyone else have this happen? Or more importantly does anyone have a cure for me?