[FunkyMelon]

The fact you got a bit of pressure releasing on cold doesn't sound good. Does the misfire go away when warm?

Generally when a head gasket is leaking slightly you'll get small amounts of coolant running down the cylinder wall when it's cold and the leak has opened, it may only be a few drops, if that. The misfire happens till the coolant has burnt off and the engine has warmed up, the gasket will seal again due to the heat and you'll be none the wiser till it gets worse.

On my old car I went 6 months on a head gasket going just topping it up till one day after a run it went completely and left me at the side of the road.

EDIT: I believe, from memory, to switch between USB and aux you use the 'mode' button the stereo.