If you've been running it a lot on LPG, then the map will probably be slightly different to petrol - even if it has been tuned, so normally the petrol ECU will still make changes to the fuel trims as it's reading changes in the lambda sensors.
In my head (and from experience on my sequential system when trying to get it set up properly - and having a Motronic P38) if you have a duff/intermittent lambda sensor then it's possible adjusting the fuel trims inaccurately (if at all) and that could be causing the issue to that one bank. Also the LPG setup has a 'bank 2' offset trim, to allow for one bank to have the map adjusted slightly to the other if they aren't giving the same LPG injection timings (I'm guessing to do with if one side LPG pipes are different length for example).
One thing I noticed a LOT on mine whilst trying to get the LPG side sorted was that if the LPG map was out too far, then the petrol ECU would keep trimming the fuel trims, and then it would start to run like shite with random misfires when it hit the limit of the fuel trim, and it couldn't trim any more.
If you're getting issues on 1,3,5, 7 - a whole bank of cylinders and that Lambda isn't changing, then chances are that's your issue - it will run reasonably in open loop on Petrol as the ECU will have a backup map for that - but over time running on LPG, if it's not trimming as it should, then chances are that bank is running too rich/lean on gas and causing random misfires.
I'd sort the Lambda sensor first, reset the adaptive values, and then see how it goes.
The other thing I've found to cause the Motronic systems grief is a non genuine MAF sensor - aftermarket ones don't seem to be made/set up to the same tolerances as Bosch ones, and this again causes fuel trims to get pulled out of whack - mine could reset the adaptives, and within a few hundred miles, idle like a sack of spanners. Putting a new genuine MAF in sorted it straight out.
Genuine MAF and Genuine Bosch lambda sensors on the motronic P38s are the 2 things that seem to keep them running sweetly in my experiences!