Update time, I've spent a busy (and cold) afternoon having a damn good look over the car. First off I got Vagcom hooked up and took some measuring block readings as advised earlier. I stuck with the misfire logging and didn't bother with the lambdas for the time being to keep things simple and try and bottom out which cylinders are giving the problem.
Earlier scans pointed to 6 and 7 (which then had the new coilpacks) and then 5, 6 and 7. The measuring blocks showed that it was missing mostly on 5, with the odd miss here and there on 6 and 7 and suprisingly it was slightly worse on lpg than on petrol. It actually logged about 24 misfires on cyl 5 in around half a minute. I'm not sure if it's my free version of Vagcom or operaor error but the count seemed to re-set itself after just over half a minute. Either way, cyl looked a prime culprit. Hopefully I've managed to attach a log file of the readings and I'v highlighted in bold the cylinder numbers in the columns across the top and also the misfire incidents under in the cyl 5 column.
I then pulled to coil packs off, starting on bank 2 and checked the earth continuity on terminal 4 of the connector and then checked for 12v with the ignition on at terminal 1 - all 8 cylinders were ok.
With the coil packs off, I swapped the plugs back to the genuine Audi ones that I put in when I first got the car. They did about 5k miles before I swapped them for some NGK's that were supposed to be better for lpg. I said that I was a bit suprised at the centre electrode on the NGK's and here's a photo with the NGK at the top and the Bosch at the bottom. You need to look closely but on the Bosch, you can see the centre electrode protrude from the ceramic insulator whereas on the NGK it's actually just inside the end of the ceramic. Looks very odd, I know the spark is big voltage but it nothing like the Bosch plug and I can't help thinking it shrouds the spark - I'd welcome views from anyone else who's fitted plugs like these, particularly if they run lpg.
I also did a resistance check on the injectors, it doesn't necessarliy prove anything but it was easier to do while I had it in bits and the resistance range is 13 to 16 ohms, 7 of my injectors were between 15.2 and 16.7 ohms so close enough for cash and one was 27.3 ohms - though this was cyl 2, which has never flagged any codes so I'm not going to get overly worreid about that at the moment.
I had a really close inspection of all the wiring, connectors etc. looking for damage or corrosion and I couldn't see anything obvious. Finally, I swapped the coil pack on cyl 5 for the one I took ofy cyl 7 a week or so ago. The codes I got after putting the new coil packs on suggested that the old ones might have been OK, so I decided to swap out cyl 5 as it had the most misfires and that coil pack could still have been the issue.
I've now just done a road test, both on lpg and petrol and so far so good. Nice and smooth and no noticeable missing. I think it logged one miss when I switched from petrol to lpg but again, I don't think this is an issue.
I'm going to give it a few runs out this weekend and see how it goes. If it's finally OK, I may well never know what the problem was. I've swapped the plugs, swapped one colilpack and disturbed a load of wiring and connectors so it could easily be any of them, or a combination. I'll keep monitoring how it goes and give more feedback as and when. I'm still half thinking on getting the injectors cleaned, given the mileage it can't hurt, but unless it gives me more trouble soon I'll probably leave that until it's more convenient to do (i.e. warmer weather).
Thanks for the assistance so far, it's much appreciated.
