Chuck a new set of the correct grade Volvo Spark Plugs in to the car before you try anything else and while you are changing the plugs take a good look at the coil packs for any evidence of tracking or even damged ends that could cause the spark tp jump to the head rather than where it is needed.
Make sure you get the plugs from the dealer for the car and they are the correct grade to run CNG, something the previous owner might not have been bothered about if they ran it on petrol all the time - Mike
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