I have a 1998 s70 T5 and it seems that every time i accelerate quickly and shift at high rpms the check engine light comes on....I checked with my mechanic and he said it was a misfire and to come back if it happend again, this is now the third time it's been on, what could it be and is it something costly to fix...

 

When was the last tune up including Cap,Rotor and wires?

 

no clue we have just purchased the car from a friend, but would that most likely be the problem and is that an expensive fix

 

Yes that would most likely be the problem.
It should be around $300.00 if you have the major tune up service done at most places.