Hi pal,
On top of the injector body there is 2 philips screws holding down a oval shaped plate underneath down the bottom of the shaft is a small gauze disc filter what blocks up and drops the gas pressure,take it out and clean, you can read the gas pressure with uptime if you have excess to it.

Pete

I have also seen this motor run about the way you describe when the "fast tas" (A.K.A. 'Idle air volume learn') was not correctly run after changing a controller or throttle body component.

yes , it is possible you have a defect lpg injector

there is a primary and assistance injector, and it sounds like you may have the assistance injector disconnected, or maybe it is a bad torque converter?

This an LPG truck? Might be a loss of pressure from regulator under load...Does this have the Nissan engine with the NIKKI Regulator? If so the Regulator shoud be putting out about 4-5 PSI even unde load,on top of the Regulator is a 17mm hex witha phillips in the middle this is the psi test port put a guage in it and run it into tilt bypass(tilt all the way FWD or Rev) and see if the PSI drops

Does the engine keep the rotation or goes down significantly?
You might have a problem in transmission or torque converter.
Check the pressure on both,and the aspiration filter.