Well if it doesn't go back I had some thoughts. A MTH that loses it's tach signal can go goofy. That depends on a couple of things though. I agree with Ted. Your issue almost sounds like a truck misbehaving for some reason. That could be anything from a broken wire, tight (wires?) so it can't swivel, etc.
First and always, look closely at the traction tires if equipped. There were a release of G scale tires that liquified when run. Then swivel the trucks and check for free full motion in all directions. Of course, make sure all wheels turn! Edit: (under power)
If you go inside spin the flywheels fully and check for binding like previously suggested.
I hate seeing engines returned for the easy stuff. I buy some second hand and find that something easy went wrong.
