my friend's 90 gxe has a very rough idle. it'll bounce from 700rpm to 1500rpm, back and forth. i did search and i've replaced the pcv valve, cleaned the iacv and acc, i checked the spark plugs which were fine although one had a little oil on it. he says he replaced the fuel filter not too long ago. should i go ahead and replce that too? once the car gets moving theres no problem. its only when it stops that its rough even if the engine is all warmed up. any other suggestions?

thanks