Rich,
Below is a couple paragraphs from another thread I copied and pasted here that might be your problem. It sure fixed all my problems with my 6 Genesis units I have.
Barry
The wheelsets have a split axle with a plastic gear in the middle. The axle ends extend into a metal bearing plate in the sideframes. This bearing plate should transfer both the electrical current and weight of the locomotive between the wheels and the trucks. The wheelsets fit into a gear tower and are held in place with a plastic plate that snaps onto the bottom of the truck. The two plastic sides of the gear tower have slots for the axles. The problem is that the slots are NOT DEEP ENOUGH. The plastic part of the axles rubs on the top of the plastic gear tower slots, causing the axles ends to “float” in the bearing plates resulting in intermittent electrical contact.
The solution: Remove the sideframes and wheelsets. Take a small round file and file the axle slots in the gear tower to make them deeper. (You might want to completely disassemble the trucks to do this. I didn’t bother since a few plastic filings shouldn’t be too harmful.) Once this was done and the trucks were reassembled the problem was GONE! This fixed all my Genesis F-units.