car idles fine and drives fine under "normal driving". if you get on the accelator, say to pass someone, car will shudder and check engine light will flash and then stay on. feels like premature detonation to me but i am no expert by any stretch. code is P0301 or P0304, misfire on cylinder 1 or 4.

where to start looking?

MAF not reading pressure correctly?
intercooler not working?
plugs?

 

I'll go with plugs.

BTW, the Volvo plugs are very good and no need to go with aftermarkets.
Stan

 

I'd also go with plugs (OEM).