Railking 30-20173-1 LIRR MU set

It sure sounds like a tach reader issue.  First step would probably be a look at the tach reader.  It could be a broken wire that keeps getting pushed together at times and allowing it to run normally, then separating and having it run away.  It could also be spacing of the sensor from the flywheel, perhaps with a loose sensor board.

On another note:

Worst set they ever made. MTH added weights under warranty to the motor car in order to pull 6 cars around o42 curves. This put stress on the gears and caused them slip. Had to modify the truck gears for better operation. Lionel set never had a problem. Really have to ramp up the remote speed past 23 to get them to perform. The heavy trucks like the subways, really hinders the performance. Over engineered, poor design.