I just bought a used 1975 Lionel U33B engine at a train show. I removed one of the traction tires which I will need to replace (the old one wouldn't fit properly and finally jammed the truck) and added new grease to the gears. I then set it to run on my 4x8 layout which only has a single loop of Fastrack powered by a CW80. It runs fine, though every once in a while (maybe every 10 loops or so) it will get really slow on the side of the loop away from the power pack connection. It goes a couple of feet... then speeds up to regular speed and zips around again and again. The tracks are clean, the connections between sections seem fine, and I doubt I need an extra set of jumpers to get the current 5 feet from the transformer.
Any idea why it might randomly slow down for a couple of seconds and then speed up? Thanks.