Help Please -Misfire

I have tried using the search feature but it does not seem to have helped me. So PLEASE can anybody offer any ideas

I have a 1997 900 which has been marvellous except for a missfire which has cropped up on occasion over the last 18 months and is now almost continuous.
My son mainly uses the car so please excuse me if I am a little vague on the symptoms. The local non specialist garage has looked at it a number of times but not been able to permanently cure it. We have now taken it to a garage which specialises in Saab, and they are unable to find what causes the missfire. I have found when I drive it, the miss is when you are on a constant throttle and that if I accelerate hard there is no problem, but when I go back to a steady throttle opening which probably giving me between 50 to 60 MPH the miss is back. The garage have investigated the electrics and not come up with anything but I would be grateful for any suggestions as it is becoming very expensive - its been in the garage for 3 days.

The mechanic who is looking at the car is suggesting checking the crank case sensor but to do this will break it when it is taken out. Anybody know what this sensor does and can it cause a miss fire?

Could it be fueling? A while ago, my son put in a tank of BP's High Performance Petrol, (Optimax?) in the hope it would help the misfire, but found that the car was worse on it.

Any suggestions would be most gratefully received.

Simon M