L322 car splutters and cuts out on gas, and now petrol too!!!

Hi, apologies this isn't more technical but I'm very new to this. I have owned my RR Vogue 4.4l V8 petrol on a 2004 plate (although it actually seems more like a 2005?) since November of last year. It is gas converted (is it zivola conversion??). A few weeks ago it started to occasionally splutter and cut out whilst on gas, but when the gas was switched off and over to petrol it carried on fine. More recently though it really doesn't like running on gas and now does the same thing whilst running on petrol. At first it was intermittent, but yesterday it happened twice and then felt like it was in sort of limp mode, if that makes sense? For example whilst accelerating from lights etc it just crawls and takes ages to build up to speed!!! Its really worrying me! It does have oil all over the front near side of the engine
compartment and recently had a new radiator fitted as it had burst and leaking heavily. Engine light comes on regularly too. At first thought it was purely down to the gas conversion but now not so sure?

Any help or advice anyone is able to give, from my description of the issue will be greatly received and help me point a mechanic in the right direction.
Cheers
Andrew

I'd be checking the PCV valves and breather hoses.....oil on the front of the engine is worrying. It could be the PCV valve is fecked causing over pressure in the oil sump causing it to blow past seals.....also check under the rocker covers for signs of oil in the pilug wells, this will cause misfires and crap running.

First thing, when the spark plugs last changed ? Gas does tend to wear plugs out quicker. when the plugs start to degrade it gets lumpy on gas but runs well on petrol..

don't go for the expensive ones, they'll only last a year.. I made that mistake !