Is the check engine light on? P1870 code? if so the problem is the transmission valve body is worn where the torque converter lock-up valve lives, transgo makes a shift kit that will fix this problem, you will have to pull the trans pan off and valve body, in order to install, has worked every time for me.

 

take the dist. cap off and see if u can move the rotor side to side as in if there is any play in the dist shaft. if there is the gear and the bushings are worn causing the timing to jump around. this happened on my 97 it would do it when u got on the highway. we hooked up a tech 2 and watched the timing jump around as the air gap on the sensor was changing from being worn. we removed the dist. and half the teeth were gone on the gear. replaced the dist. and has ran great ever since. before checking this i replaced everything from plugs to ignition module. and still nothing. check it out wont hurt.

 

I just had the same problem with my 99 blazer and it was the crankshaft sensor that was bad. I replaced it and now everything is fine.

 

What about bigger tires? Are you running bigger tires? I think that's the culprit for my RPM's jumping around on the highway. It acts like it wants to shift up then down then up sometimes at certain speeds.

 

I have the same problem in my 98 blazer. I havent fixed it, but I thought it was the throttle position sensor, since the symptoms are similar. But now i know it could be any of these things. Wow