bad gas wouldn't cause one cyl to missfire.

Just pull and replace plugs. not familiar, but does your motor have coil packs or a distributor?

If it does have packs, pull the coil and test it with a multimeter or take it to autozone where they'll bench test it for free.

I had to replace two coils on the bimmer and the feeling is very similar to having a bad/fouled plug.