I have a 2002 SMG with 108k miles. I have not had a problem to date (knocks on wood)--there was also no indications of problems in the car's service history. I don't follow what you are saying, the car slips out of 4th and grinds (not good)? Or do the rpms surge when you give it more gas (clutch slipping)?
I am assuming that the clutch is slipping. This does not require a new transmission and could be as simple as recalibrating the clutch adaptation--which has to be done with a GT1 or somethign similar. You can also do a 'soft' reset of the clutch by accelerating to 45 mph, putting the car in neutral, pull both paddles on the steering wheel for 10 seconds, then put the car back into gear and carry on as normal . You will not get any feedback from the car to tell you the reset happened, but it will.