The code is a P2138. The two accelerator sensors inside the accelerator assy, are out of sync. That truck needs a software update from your local Clark dealer.
The tilt issue: Under the lift pressure hose that comes out of the hydraulic control valve on the main valve section, is a plug, behind the plug is a small "poppet" valve and spring. Remove the plug, be careful not to lose the spring, remove the poppet valve and polish it with fine crocus cloth until the valve moves freely inside the control valve housing. Clean the poppet and the bore in the control valve and reassemble.
It has a juniper propane evaporator and drive by wire throttle if that helps.