Cd's will play fine as long as your parked with the car off, as soon as you go to leave the cd stops and the screen reads com error... And twice the radio has abrubtly cut out and came back on, like it shorted...

Does this have anything to do with power being cut off to the vehicle - one you have opened the door?  In various vehicles of mine, the stereo will stay on when ignition is switched off - but shuts down when the door opens.  The stereo stays on with ignition off - in case you parked / turned off the car - and remain in the car.

this is definately a short etc in the head unit... I can almost make it short at will by swerving back and forth, the cd playback is odd though, it may play for 2 or 3 songs and then it will stop and show the com error on the screen... It only acts ignorant when driving though...

I added a CD changer to my C5 ... it had similar situations ... the issue resolved itself with a specific way of plugging in the unit to the wiring harness, then inserting the CD cassette.  Is your CD changer OEM, and factory installed?