I'm no car expert at all, but i had a similar problem a few years ago on a V6 Manual Vectra, and it turned out to be a Planetary Gear in the box.
I would suggest you get it to a gearbox specialist and get them to look at it. The box is likely repairable, but if you keep working it you will probably need a new box..and i assume that will not be cheap on a Saab.