My 97' blazer surges in the warmer months of the year. It also happens after driving over 50 miles or so. Whatever it is seems to be temperature sensitive. If I give it more gas it will quit momentarily. It happens around 45 - 55 mph. I can't activate cruise control at that speed because of this.

 

This speed range is indicative of a torque converter clutch shudder. Try searching for "tcc+shudder" and see what explanations turn up.