Slight misfire - plugs, coil or injector?

[antimony]

2006 Mercedes V8 M113 engine, has a slight misfire under low load, has only happened when cruising on the motorway (70mph @ just over 2k rpm). When it happens it obviously shuts down that cylinder and is very lumpy until I stop and restart. My primitive code reader identifies it as cylinder 1 and I can clear the error. It hasn't occured when on petrol, only gas, and it is fine all other times. The only other symptom is a very slight flutter on idle (petrol and gas, more noticeabe when cold).
I initially thought it was a gas injector but the slight idle problem has made me think it's the ignition.
The plugs have been in since it was converted - just over 30k miles ago (MB recommend 60k replacement intervals).
So my question is; where to start? It seem silly just replacing the spark plugs on that cylinder, however it has 16 of the things so replacing them all is expensive.
A coil pack is £50, but again it may be unnecessary.
Or take in to an LPG garage - possibly the most expensive option.
What to do????

[Gilbertd]

Sounds like you are on the right track. LPG needs a better spark than petrol so any weakness in the plugs or coil would show up first on LPG than on petrol. Take the plugs out on that cylinder and another and compare them and try swapping the coil pack to another cylinder and see if the misfire moves with the coil or stays on number 1.

[superkp]

my v6 would miss fire it was air getting in where the holes were drilled on inlet manifold ..check and see if its loose