Engine troubles, just started

[Left4james]

Hey guys, just went out for a little blast in the Passat and after about 30-40 mins it developed a weird 'mis-firing' sort of problem.
In the lower rev range it is spluttering, once over about 3k it seems fine, but anything under it is juddering and you can feel it trhoughout the car. Also took up the bonnet to have a look and it also came with an intermittent bang under the engine cover. Like a party popper going off almost.
Its a 20v 1.8 non turbo.
Any ideas?
I am thinking it is likely the disto cap or maybe leads and sparks.
Thanks guys