I had all sorts of problems with my LPG system. Seemed to run fine on petrol, so I naturally thought it was a problem with the LPG. Had it in with multiple LPG mechanics who tested everything, only to be told the system is running fine, mixtures were all good etc.
Then one day, my regular mechanic told me that my PCV hose was split. I replaced that, and all of my LPG problems went away. Turns out the engine was sucking in too much air, which didn't seem to worry it when running on petrol, but on LPG it made all the difference....