Range Rover p38 4.0 V8 Doing my head in PLEASE HELP !!!

[L4LEYDAVE]

Hi, I have a 97 RR p38 4.0 with bedini LPG system, If i start and run on petrol all is ok (maybe a little under power but not too bad). When I click onto LPG after 20 seconds the screen is coming up "gearbox fault" it then clonks into gear (ITS AN AUTO BY THE WAY). I had i plugged into a computer last week, i was advised to change the alternator and one of the lambda sensors, also to add an engine and gearbox earth strap which i have done, none of which has slolved the problem. Can anyone PLEASE help. Dave.