Classic misfiring when engine hot

Hi guys
Hope you can give me some advice on my Classic 1992, 3.9 V8, lpg/petrol.
It's got lpg so within the last year I've fitted

A&R Power amp, ignition amplifier
Magnecore HT leads
New rotor arm
New coil (last one died)

My Classic has recently developed an interesting problem.

I have to go out on a saturday morning and drive 22 miles which car does no problem, however when I drive back home towards the end of the journey say last mile when I hit a bit of traffic it starts to misfire to the point that if I put my foot down it spits and bangs, if I gently press the accelerator I can creep along but no hard pressing.

I thought it might be the HT leads so switched engine off wiggled leads and restarted and it seemed ok but then a few mins later same problem will happen. If I swtich off and back on then it's ok for a while.

I then leave the car for a week and chance it again, the journey down is fine but back it happens again.

I did a 30 mile journey during the week and it was fine for the whole journey.

I've tried switching between lpg and petrol but no help.

What I can't figure out is why it's happening when the engine is hot.
I thought it might be fuel issue but then surely switching between fuels would resolve this.

I hope someone on here might have some ideas on how I can locate problem as it's driving my daft and makes me a bit worried to do long miles as don't know when it's going to happen.

Many thanks

Dr W

Try replacing the rotor arm with a new genuine part first, or you can try removing the dizzy cap, pull out the HT lead from the coil at the dizzy cap and holding it about 6-8mm away from the top of the rotor arm with a pair of well insulated pliers, get someone else to spin the engine over briefly. If a spark jumps across to the rotor arm during the process then the rotor arm is shorting down to ground and needs replacing.
If it is backfiring, that is cause by unburnt fuel in the exhaust which points to an ignition, not a fuel problem. Another possible cause could be a burnt out/leaking valve and a compression test on the cylinders would indicate this.
Try the rotor test first-aftermarket rotors can be suspect,

Cheers for reply

Will look at rotor arm issue first.

To confirm you are saying remove dizzy and hold coil HT lead near rotor arm and check for sparks jumping across to indicat shorting. OK will check.

Whilst replacing the other ignition components I didn't replace the dizzy cover, perhaps it needs replacing.

Is there anyway to tell if there is aproblem with the valves without doing a compression test?

Cheers

Dr W

Yes, you have the procedure right. Wouldn't do any harm to change the cap. Normally, a sign that any valves are bad is that the engine will run "Lumpy"-more noticeable on a 4cyl. than a V8. Realistically, the comp. test would pinpoint it better as deviation from the average by say, one dodgy cylinder would stick out like a sore thumb.
Check/replace the cap and rotor first and let us know how things go. If it turns out to be the rotor, be sure to buy and fit a genuine (Usually Lucas) replacement which is not that more expensive than a spurious.

Afterthought-I take it you have done the obvious and checked the spark plugs???????????

Haven't as yet only because it runs fine the majority of the time and only misfires now and again which made me think it couldn't be the plugs.

I plan to look at it this weekend but wanted to get a list of things to try before I started.