p38 help on tickover and gearchange

can any one help me i have got a p38 4.0 auto with lpg
the problem is the revs are just under 2000rpm on tickover and when i put it in to d the revs drop to 1000rpm, at 50mph taking light pressure off the accelerator it changes down gear and revs go up to 4000rpm and it keeps doing it any one help cheers

think this (or a similar prob) was recently posted, sounds like throttle position sensor