up-down,up-down,up-down shifting

I have a 2000 silverado 4.8 extended cab short bed pick-up with a shifting problem.

At 50 m.p.h., it keeps shifting up and down. It is only about 200 rpm difference, but it is annoying. Any speed above or below that it is normal.
I just noticed it when i put a flowmaster 40 on. The tack goes from 1500 to 1700, and back over and over.

Could this be a sensor of some kind? And if it is can I replace it myself?

Thanks for any help

In these trucks there is a low and high gear for overdrive. it is suppose to keep it from downshifting to third to help with MPG. so it sounds like a sensor like you said or something with the tranny.

i have a 2000 4.8 ext cab short bed also. about a year ago it started doing the same thing. 2 months ago had to replace tranny. it didnt give me an error code till 3 mths before it quit. it gave the p1870 code. tranny slipping. i got in it and lost reverse and 2nd and 4th. good luck.

that sounds like a clutch when out in the tranny. I had a 1979 Monte Carlo that lost Reverse and 4th. Clutch went out

One thing to heep in mind is the engine has to be running up to par for the transmission to work right.

Check engine tune up first.
Check for codes and run a scannner check on transmisson and engine?
Has the transmission ever been serviced.
How far back on the exhaust system was the flowmaster installed?