Bi-polar 4r100

[crash7.3]

Most of the time if I keep my foot out of it, my trans shifts perfectly. But periodically, usually after my tranny is up to temp and after some moderate accelleration, my o/d light comes on and it will not shift into 4th or lockup. Also, reverse begins to slip and jerk. The truck still has the original trans as far as I know, and the fluid is still red and not burned. Anybody know what's going on?