Have any of the "techs" connected a diagnostic service tool to the forklift to see if any codes are stored?
If nobody has connected a diagnostic reader to the truck I wonder how it was decided to blame the APPS?
Anyhow, if you did not try to "center" the mounting screws of the new APPS when you installed it, you may be preventing the ECM from learning the true idle position of the APPS.
You say it is "coding out". Do you mean that the amber malfunction indicator lamp lights up?
I would be interested to know what DTC code is stored, or active, and a diagnostic reader is best for that.