My son and I are beginners with an HO set but have a problem. We have some track laid out on a 4x8 board. The current layout is a long oval about (4x7) with a smaller internal loop, including a powered switch and a manual switch. The track is not nailed down. When the set was new, the locomotive would start and run great. Now, it is erratic, and "jumpy", sometimes requiring a slight hand push to get going, but it does not run well. We have the same problem with two different locomotives. We have tried a new power supply/controller, but get the same result. We even took the locomotive to a good hobby shop and the repairman said there is no problem with the motor. We have used a track cleaner on the rails. What should we try?