Tonight while running on lpg sat in traffic my RR started misfiring on &
off then seemed ok. Then traffic moved and as I tried to move off it
started spluttering, almost stalling. I quickly flicked over to petrol &
it was fine. A few more 'tests' on the journey proved that it is only on
gas that it happens. I'm having to top up the rad with coolant as the
dash light keeps coming on - I've pinpointed that to the expansion tank
cap- I can hear it escaping when hot, so I'll get one of those in the
next few days. The engine isnt overheating but I wondered if the two
things could be related seeing as the pipework flows through the gas
assembly. I know the gas level indicators are usually a way out but I'm
easily 60/70 miles from being empty (I use the trip meter to guage it)
There are no obvious faults (hanging wires etc)

Or any other ideas anyone please.

Dave