If this is an automatic transmission S-10, then the problem you describe is more likely to be transmission related than engine related. If the torque converter clutch is disengaging momentarily it will cause teh RPM to surge around 100 RPM just as you describe; also this will not happen untill the torque converter clutch is commanded on (after the engine is warm and the vehicle goes above about 35 mph).
You can find out if it may be a torque converter clutch problem by lightly applying the brake pedal when the problem is occurring; not enough to slow the truck but just enough to make teh brake lights come on. The brake light switch is a computer input that causes teh computer to command the TCC to disengage. IF lightly touching the brake pedal causes the surge to stoip when it is occurring, then the problem is in the transmission, not the engine.
If this is what youa re experiencing, youc an try ahving the transmission flushed/ all of the fluid replaed at a shop that perfroms transmission flushing. Replacing the fluid with fresh new fluid usually helps resolve this condition. Also, there is a product called "shuddeer fix" ; it is available through NAPA parts stores and probably others. A bottle of this added to the fluid (after it has been flushed) also helps in stubborn cases.
If this does not resolve the problem, replacing the torque converter may be required for repair.