hey i have a 2000 s10 blazer 2 door 4.3 v6 and auto my problem is cruising around 55 my rpm's jump up and down 200 rpm's theres no check engine light i cant really feel it but i can hear the engine changing rpm's so i dont feel its a bad gauge

 

Sounds to me like the torque converter clutch is locking and unlocking repeatedly. It is quite possible that the clutch itself has gone bad. However, it could be a failure of the TCC solenoid in the transmission.