I recently bought this 1998 s90 Volvo the previous owner has put a new fuel pump and spark plugs 3000 miles ago ,yet the car still has issues ,the car starts to violently misfire at 50-55mph and wont achieve greater speeds. The engine also kind of shivers once in a while when idealing it will dip for 1 second then return to normal ideal rpm‘s. Does anyone know what may cause this issue? ,and or how much it costs to fix?

Thank you very much
-Z3

I talked to the previous owner and he gave me some info and said he had it diagnosed and that it would be the cam position sensor but the job it self would be 500$ altho a reliable sensor only costs 60-80$.Maybe it is because there has to be some timing that has to be adjusted or some other tune ups but ill first get that sensor fixed then keep yall updated.