Misfire

I've been having trouble for two months with misfiring. My RR is a 4.0 2001 HSE converted LPG.
It mis fires on both petrol and LPG but mostly on petrol.
What we've done is replace the lambda probes. Coils, Leads, and plugs. Also replaced the crank shaft censor. Now as the management system rarely goes wrong can anyone suggest anything else it could be.
When reset it all runs fine until I switch her off and when I get in again it either runs on petrol and not gas or the other way round.
When I say not run it misfires and would not pull to top of a rice pudding.
Gas system has been serviced and nothing wrong with it.
I do hope some body has the solution.

maybe water getting in the bores ,any signs on any of the plugs you removed

Reconditioned engine fitted 1 year ago so I hope that it is not water in the bores.

I had the same problem. my man is busy now taking the head off and re grinding the valves in.They were pitted and nasty also the plugs were manky. Hope it helps