combustion temps of LPG are higher than that of petrol isnt it? that could've led to a melted catalyst. you'd need to remove them and inspect but these cats hold up ok with forced induction so i dont think LPG wouldve burnt them out.
plugs should be chosen to suit LPG too, to avoid misfire and preignition issues.
i'm speculating a fuel supply problem. some of the aussie cop cars that ran on LPG were known to backfire or stall, esp under heavy acceleration or when the engine was cold.
but you say the problem persists with petrol too? after you fill up with petrol, have you tried resetting the ecu (battery or pedal dance method), resetting the learnt mixture settings (disconnect a maf sensor with engine running to trigger a fault then clear it) and also driving a few tanks of petrol so it readjusts?
there are some faults you mentioned which rings a bell. have a browse around the forums here and you might find some leads
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