Try unplugging the transmission kickdown switch in the '84 300D. A short in this circuit seems to be fairly common.
http://www.peachparts.com/shopforum/showthread.php?t=128500
Unfortunately, I think that it's unlikely to be the source of the problem in your '85 300D since you'd have to floor the accelerator to activate the kickdown switch. I guess that it's possible that the switch is stuck closed and the circuit is shorted, but that seems like a long shot.