I have the same problem with mine, it has gotten progressively worse. It is especially prone to doing that when I freewhweel downhill. Luckily I have another O.D. trans I will rebuild before installing. If I remember correctly someone on this board came up with some good ideas for potential problem areas to explore. Also someone else on this board provided the following url that should be extremely helpful: http://www.classictruckshop.com/clubs/earlyburbs/projects/t85n/rebuild.htm