'99 Isuzu Trooper, 97k: RPM surges when between 2250-2750 rpm only when speed is between 55-65 mph. Timing belt is original. Transmission sometimes late to shift from 1st to 2nd or 2nd to 3rd, when just starting out on drive. Do I have a timing problem, transmission problem or something else?
My first thought for the surging is a worn out spot in the throttle position sensor.
It also sounds like the transmission needs a fluid and filter change, that could explain the slow shifting when starting out.
Im not sure what your vehicle maintenance schedule in the owners manual says, but you could be overdue for the timing belt. It probably wont show any warning symptoms, itll just break sometime, probably at the worst possible time and place. If your engine is an interference engine, it could tear up engine parts when it breaks.
Try one of the engine intake cleaners. The directions are on the can. Some people are reporting that it helps for surging.