700r4 I'm running has started shifting back and forth from 4th to 3rd to 4th and back to 3rd when cruising along at 60 mph. Goes into 4th when I hit 55 but have to either run slower or simply shift to 3rd and run in that gear to keep it from doing that.
Any ideas on what may be causing this?

 

 

Fluid is clear but that's probably because I changed the filter a couple of months ago and haven't driven it much since.