Misfire when hot

Hello everyone.
After taking care of the usual faults, my 1996 P38 4.0 has been running like a dream for hundreds of miles.
A couple of weeks ago I noticed it 'popping' when it is started from cold. This clears up in seconds and disappears completely when the LPG cuts in.
Everything is OK until about 10-15mins into my journey. I do a lot of motorway miles and usually stick to 70-75MPH as need my license for my job !
The car drops a cylinder when I try to accelerate and will not do more than 75MPH. If I switch to petrol it makes no difference.
Occassionally I get another 'pop' from the exhaust and the eighth cylinder comes to life.
It seems to 'hold back' and distinctly lacks power, even when the gearbox drops a cog.
This problem used to be rare but is getting worse.
Ive changed the plugs and the leads for quality items but there has been no change.
The LPG is fully serviced and the engine was service 500 miles ago.
All vac hoses have been checked (replaced one) but still getting problems.
Car starts OK and revs easilly. I use it daily but want to eliminate this problem.
I'm currently running some REDEX through the petrol system but its doing nothing.
Can anyone tell me what could be wrong or what should be on my check list next.

All replies gratefully received

hi get it hot and unplug the maf senser, get some body to sit in it and keep her running, then take it for a run and see if any diff

you need to establish or explain a bit clearer, is the problem hapening on both petrol and lpg?

i reckon one of the sensors, crank/cam/maf

double check the coil pack, only 4 small bolts to take it off, clean the terminals.

take the plate off the engine side of the gearbox at the bottom (not the small round one) you will see lots of 'tabs' all the way round the flywheel make sure all are nice and straight.

check crank sensor is not damaged if any tabs are bent. check its in nice and tight when done.

i dont think it will be injectors if its happening on lpg aswell as they are turned off and emulated

To explain in more detail:
Start car from cold and revs OK (car always starts in petrol)
Whilst on petrol, get 'pops' from exhaust between 1500 & 2000 RPM but revs OK 2000+
As soon as car goes onto LPG (after about 2 mins) popping stops
After about 15 mins of motoring, top-end power drops off and cant get past 75MPH without engine 'popping'. If I back off the throttle slightly it seems to pick up (weird).
If switch to petrol at 75 MPH get a loud 'bang' from exhaust and car sets off like a rocket but loses power when go back to LPG.
I would normally start looking at the LPG system but it was recently serviced and this fault is present one day and gone the next.
Ive renewed plugs and leads and checked for any stray sparks from the coils or leads at night but fault will not clear.
Fault seems to be on both petrol and LPG as car will sometimes struggle to pass 75MPH on petrol. Suspect the fault is more evident on LPG due to the inherent loss of power that comes with using LPG.
The garage that MOT'd the car 8 months ago said he had a 'bugger of a job' getting it through the emissions test. Could this fault be oxygen sensor ???
Will check MAF when get a chance. Flywheel tabs and sensor were OK last time I checked but will double-check when do MAF.
Does anyone know the range of the sensor ? (thought it was about 3mm).