Check the electronic throttle module, and wiring to the pedal, these i have found to be problematic, sometimes it would rev and othertimes the rev range was limited, and then it would run at a fast idle with no pedal input, issue was faulty module on one fork and pedal sensor issue on the other, price on genuine parts far exceeded our labour and modification costs, customer was happy to go with our solution.
We ended up removing the electronic throttle control and went back to a conventional cable control foot pedal on both forklifts.