I have a shifting problem with my "98 chevrolet C1500 wt, 2wd, 4.3 liter. automatic.Shifting is fine until it goes into overdrive around 45 mph or so but then it downshifts and continues to go back and forth and is ok above 55 mph. The tran fluid level is ok which was changed about 20k miles ago. the truck has 146k miles. Any suggestions? Thanks for your input.

 

I'd spend 15 bucks on a modulator valve, and vacuum hose.