We have the same problem. CG33 G424FE engine dedicated LPG. Bought a few months ago with this issue.
Heaps of stored codes from CO2 sensors to temp sensor etc.etc.
Runs ok cold and backfires when up to running temp.
We have changed spark plugs, leads without any change.
We carried out a leak down test and had a horrible creaking noise, starting with almost full leakage and once the noise ceased, had full seal.
i noticed oil vapour coming from around the valve spring area and was watching the lifter bleed off as well. our cam is showing signs of breaking up on the full lift point. From experience this usually is from constant pressure on the cam.
I am leaning towards bent valves-probably from a mis-timing or worn timing components or the lifters are pumping up.
I am about to order gaskets to do a head R&R and see what that brings up.
I will keep in touch as we go.
Parts are expensive compared to normal passenger cars in a similar configuration. $280 aud for a Thermostat and $190 aud for an Inlet valve.