Citroen Berlingo misfire

Citroen Berlingo 1.4i LX 600 LPG. Bought 2 years ago with 28K on clock. From day one has always had a slight misfire. 4 coil pack failures no garage able to detect the underlying problem until now. Citroen dealer says they need to strip the engine. My local Autogas specialists suspect the ECU. Last garage visit showed random intermittant undetermined misfire on one or more cylinders - coil pack changed - retested - fault showing was permanent random undertermined misfire on one or more cylinders. Coil pack fault had cleared. Revs on idle constant. Another garage recently gave me a fuel cleaner additive (trade product) of which I've used 1/4 of a tank. Engine now dips below 800rpm (almost to stall) on idle and tends to hunt - not present before. Depending on the run on occasion does sit at right engine idle revs and feels smoother on misfire but this is intermittant - misfire still present. No reports about bad compression injector problems or sensor failures. Over the past year the misfire has caused excessive engine shake/twist - both bottom ball joints worn and replaced - all engine mounts worn out and replaced - steering knuckle joint now has a lot of movement/play.