Clunking/Thumping noise P38

I have recently purchased a 4.0 P38, and occassionally when pulling away with the engine cold, get very slow/non-existent acceleration and a loud clunking noise (first time it happened was convinced something was stuck under the vehicle!) especially when attempting a hill start.
Either letting the vehicle idle for a while or repeated kickdowns of the accelerator appears to sort it out. It never happens when vehicle has been running for a while.
The noise is coming from the passenger side of the vehicle - approx midway along the vehicle.
The car has an LPG conversion.
Does this sound like gearbox problems or something to do with the LPG?

CHECK ALL YOUR EXHAUST RUBBERS ARE STILL IN PLACE.
SOMETIMES IF A UNDER CAR LPG TANK IS INSTALLED THEN THE LPG CONVERTION FITTERS MAY HAVE ALTERED EXHAUST MOUNTINGS AND THIS COULD BE RESONATING ON UNDER SIDE OF CAR.

JUST A THOUGHT AS I HAD A SIMULAR ISSUE WITH A GRAND CHEROKEE 4LTR THAT HAD GAS.( BAG OF **** IT WAS).