V8 Misfiring on gas

My 3.9 has a slightly irritating issue that makes it splutter when hot and running on gas, its fine when engine temperature cold-warm but when you've done a 3 hour run then hit some traffic its a bit spluttery when your pulling away almost as if the mixture is to lean, however I think its more likely an ignition issue. At speed it is fine, just the initial pulling away from junctions etc.
It runs like a dream on petrol and recently passed emissions test with flying colours.
It has NGK BPR6ES plugs, Lucas Leads, cap and rotor arm and an intermotor coil, all fairly new, it is an omvl single point LPG system with lambda feedback.
Would it be worth trying a different grade of plug? or worth getting genuine LR Dizzy cap etc. (Aren't they just lucas at 4x the price?)

Any help much appreciated,

have you checked all the LPG pipework going from the vaporizer to the plenum chamber?

If its anything like mine you can adjust a screw on the vaporizer to make the mixture leaner/richer.

Cheers for reply, don't think mines adjustable though, anyways it was fine for 4 years I've owned it until recently, just had a thought though - I might check pickup air gap in the dizzy, haven't ever done it so worth a check.

I found that I would burn up points and condensers At least yearly, if not 6 monthly when I had LPG on my old Range Rover classic. I would keep a couple of spare one in the car at all times. First signed were miss fires under load. Then misfire while cruising. So would change condenser and points. I also found that I had to set the dwell "by ear" as the distributor arm etc was worn and if I set it to factory spec it would still miss. I ended up using a dwell meter and through trial and error found the righ dwell and marked it on the meter for next time.
Hope this helps

I don't trust anything not SGI or better.

But I reckon it will need recalibrating