Volvo: Volvo S40 misfiring, oil filler cap, system checks

Question
I have a 1996 S40 1.8 petrol S40, mileage 102000.In the last year it has been misfiring more and more, none of the warning lights come on and it can't be the fuel as I fill up at a different garage each time. Some days it will not misfire at all,others it will usually on acceleration. The length of journey does not seem to matter. The only visible difference in the engine is a brownish sludge on the inside of the oil filler cap, but the oil has been changed at 100000 along with the cam belt. Any ideas?

Answer
Andy,
Misfires are mostly from the ignition system. Check the plugs and related parts such as the coils or plug wires to see if there is a failing part. The fault code might indicate what cylinder is misfiring, start with that cylinder. If that system checks out see that all the injectors are plugged in tight and firing. That would be the second most likely misfire cause.