S8 with bad random missfire.

[Daedalus1]

I have been getting a very bad random missfire on my S8. It will go weeks with nothing and then it starts to miss when starting up or just driving. I have had the plugs changed as they were coming up due anyway, I have put on two new Power Output Stage modlues after the first one (fitted by my local Audi independent specialist did not seem to make any difference) It went well for some time then started missing again. On VAG Com it comes up as a random multiple missfire on various cylinders on both banks but not necessarily all of them at the same time. I am beginning to suspect a broken wire somewhere but where to start looking????

Have any of you fine people any thoughts on this? I dont want to use the independent again they were expensive enough and did not sort it. The local Audi dealer wants £115 an hour, something like this I just dont trust them not to wind it out, if it takes a day thats a grand nearly.

By the way the car is on LPG and has been for the last 55,000 of its 175,000 miles. It misses on both petrol and LPG

Thanks in advance.