99 blazer 150k. Trans done at 105k. the issue i am having is after i am at highway speeds for longer than an hour. when i get off the highway and driving about 40-45mph, rpm at 1500 the tach rapidly fluctuates between 1000-1500 rpm and the trans feels like it shifting back and forth. i don't know if its mechanical or electrical. Any info would be helpful

 

MAX WEDGE ? That's a Mopar engine ! Nice - I had a Coronet 440 back in the day
Your trans is a 4L60e ? Sounds like the torque converter is not staying in lock-up mode ... check all the usual suspects - fluid level , color , smell ....
Then look at Your throttle position sensor

 

fluid level, color, and smell look good. How do i check TPS ?

 

You would use a digital multimeter to back probe the leads on sensor to check operation . The TPS is a potentiometer in produces a low voltage reference signal of less than a volt and should ramp up smoothly depending on throttle position to WOT or wide open throttle , around 5 volts . Test with key on-engine off . Voltage (displayed on meter) must track throttle position movement ... NO dead spots or sensor is shot . I use a good fluke meter to test , but I have used junkyard pulls in the past to test with ... got a drawer full now