I am working on a 8026 now and have a similar problem. It seems the roller pickups are bad and not conducting electricity properly. I have cleaned them thoroughly to no avail, and it seems like replacement is the only option.
Connect one transformer lead to the metal chassis and the other to the brass strip underneath the loco. The loco should run properly. If it does, touch the roller pickups. If the loco does not run now, they are bad.
The part number for them is 8209-165 available from parts dealers.
Brasseur sells Lionel and K-Line parts.
http://www.traindoctor.com/service/catalog.aspLarry