Could be a couple of diffrent things....could be that the carb power adjustment is out and needs readjusted.....i have seen the same issue on several trucks with your exact complaint being the carb out of adjustment......
....could be a transmission issue with pressures/slippage...would need to have a QUALIFIED forklift technician do a transmission pressure test to see if the is a issue with transmission....
......could be a brake hanging up and dragging....
...lthis is just my WAG arm chair diagnosis....and just because you have had someone look at it already,dosent mean they are correct or know what they are doing.......and just because i commented dosent mean i am correct.....