I have a Corpet, bought S/H, which runs badly. I've checked the track & pickups, which seem OK. Track clean; other locos work OK. The problem can best be described as 'lumpy' running. At a constant controller setting the loco moves slowly for about half a wheel revolution then quickens for the other half, then slows again etc. Upended and power attached to the pickup shoes, it seems to run normally. I was wondering of the two driven axles are out of sync, and need re-quartering. Can anyone advise?