I'm assuming that this is a 347 series truck.
If so, we had the same issue with both an original controller bought from Linde as well as a controller that we purchased from Flight Systems. We spent days on trying to resolve this issue trying different controllers, replacing the accelerator pot and on and on and on..
Although the default setting is quite different, try setting function 11 to 51. That is the only way that we could get our truck to attain full speed.