1990 Saab 900 Saab 900 Odd Behaviour

I really need some advice here, I just bought my daughter a 1990 Saab 900s, it's not a turbo. When driving along it seems to do this odd surging type thing, it's sort of intermittent if you let up on the accellerator and slowly engage again it will gain speed, if you don't let up while it's doing it, the problem continues, It won't increase in speed and it does this surging type thing. It's hard to tell if it's engine related or transmission related. Any suggestions would be greatly appreciated.