From all the info you've given, I'd say it was a problem of excessive vibration getting to the CD unit - I presumke that it only does it when teh car's moving?
Being an older unit, its performance in bumpy and vibrating environments won't be too good, and this can be made worse as the laser gets old and drops off in intensity, and/or loses its ability to track as closely as it should. There's no real easy fix for this, but you could;
(1) fit a stock 138
(2) try to soften the mounting of the CD unit. If it's directly mounted to the dash, is there a way of 'decoupling' it, eg by making up a tray lined with foam that it can sit in. Mount the tray to the dash, and sit the CD player in it. Another option would be to try to use rubber grommets on the mounting screws/bolts, and not tighten them up too much. To see if this will work, you might be able to temporarily remove the CD player from the dash with wiring intact, and sit it on the floor or a seat while you go for a drive.
(3) sting the 'girls' for $50 each, and buy a new unit
If it's doing it while the car's not moving, and/or the engine's off, this would point to an internal electronic or an external wiring problem, or maybe even intereference from your ignition system if it's a powerful one.