S3 Jumping out of gear

I am considering buying a S3 109 Station Wagon (diesel), but the seller says that it jumps out of 3rd & 4th gear. Any idea what might cause this, and is it a big problem to fix?

Difficult to be certain; it might be as simple as a weak detent spring which can be fixed easily, or possibly a worn selector fork which again can be fixed from the top without taking the gearbox out. BUT, it could also be wear inside the box which is a big job, gearbox out to fix. Are you mechanically competent? It isn't too hard for a competent person to re-build a series gearbox and most of the parts are available but will cost you more as you are in Norway. You will need a parts catalogue.
PS: Just thought of some other possibilities: Loose mainshaft rear nut, or too much end-float in the mainshaft, (which can be caused by the aforementioned loose nut).

Change the detent springs-cost very little-you might as well replace the balls- cost pennies but you are sure to lose one.
probably ie most likely, you will need to rebuild the 'box- haggle some more.

Probably a 3/4 syncho and the meshing gears, about £200. the 3/4 synchor has a very slight angle (a degree or two) on the dogs to hold it in gear when the torque is on, once they wear parallel it will come ouf of gear. The detents will help but once the angle wears the force will eventually overcome it. Its a simple job - box out, strip and replace those parts and its worth budgeting for as you can do the seals and clutch and release bearing at the same time and odds are some thing else will need doing, but its a hard (physical) weeks work. I think you have to take the view that whether it jumps out of gear or not, if the box has not been stripped for 10 or more years it probably will need to be soon, even if its just the seals. Get some money off and go for it. Take the view that every series you look at will end up needing a box or engine strip, but some will be more honest and reflect this in the price (lower) while others will not.

Yes, good advice. but if you have not removed a series gearbox before, you will be surprised how much fun it is.....

Except for the smell!