Did you try to disable cruise-control using the Bell and Horn buttons on the train transformer? This takes the PS2 tach circuit out of the system.- If still jerky at slow speeds as you ramp up the throttle then most likely mechanical issue.
If you do go the unpowered route I assume you mean as a dummy unit dragged by a powered unit. If you keep electronics installed and remove the motors you would still have lights, smoke, and sound. The headlights would be directional if your unit has that capability, the smoke level would be fixed at the idle/minimum level, and the engine rev'ing sound would be fixed at the idle level. But since you would have another powered unit with presumably operating sound you could just turn down or mute the volume of the unpowered unit if it is distracting.
OTOH if it is a mechanical issue that you don't want to chase down, I'd think you could remove the electronics which is presumably functional and sell it particularly if it's a PS2 3V version. Then if you really want lights and smoke there are simple/inexpensive ways to do that.
