I'm glad you found the problem and it was one of the items i was going to suggest - the plugs. Iridium plugs are all very well on engines originally designed to run on them but regardless of fuel, tend not to give improvements on other engines.
Second thing i was going to suggest was the fuel mixture which is where your meachinc was in the right area too. If the mileage on the car is more than 100k miles (160000km) and the Lambda sensor has not been changed yet, i would do it as preventive maintenance - their service life is ~100k miles (160000km) or 10 years.
Petrol is easier to ignite than CNG/methane or LPG so two things come into play here - the engine is more tolerant of minor mixture differences on petrol and secondly, the spark has to be more precise on gas (any gas) so both of these scenarios point to Lambda sensor and plugs.
With your plugs, i'd suggest using Volvo or NGK standard plugs but decrease the gap by 0.1mm and change them when it starts to misfire (your push/pull - it "holds back" as a result of the misfire) for new plugs, again with the gap set 0.1mm smaller.
Lambda sensor, if it's never been changed and your car has more than 160000km, renew it. When you do, reset the fuel ECU fuel trims to standard, they will have wandered from original to cope with the older sensor so they need resetting to factory so they have a clean base to relearn from.
I hope that translates for you, good luck with it!
