I recently purchaced a 00 s40 it has 70k mi. under heavy acceleration it has a signficant missfire
it wasn't bad until yesterday and the check engine light came on
the code that the car gave me was p0300 and a cyl #2 missfire
the #2 plug wire was bad
i changed the plugs with bosch platinum +4 and volvo o.e. wires
i reset the codes
the missfire isn't as severe but its still there only after 4000rpm
also @ idle the car vibrates pretty bad, but not in reverse, i've heard that is the pass. side engine mount

I would replace both of the coils they are known to go bad.For the vibration replace the L/S engine mount they are also known to be bad.

Hi
I had the same problem on my v40 2.0T and it turned out to be one of the coil plug leads which are only availiable for Volvo, I got away with only buying one and when it wasn't cured on the first cylinder then swapped it onto the second and it went away. the coil in the plug lead drives two plugs.

Simon