Hi Mark
I wouldnt cosider myself any sort of expert, but i think i might have the same problem and believe i know what the problem is. Let me ask a few questions first though.
1. How many Miles does the car have on the clock?
2. do you know if the clutch has been replaced?
I had this problem with my Mk3 gti and now with my Mk4 gti as well. I believe its the clutch release bearing. When your on the motorway at say 70mph in 5th gear and you bury your foot in the floor, do you revs shoot up without any power or does it do what its supposed to and take off like a rocket? If your revs shoot up without any power gain, then your clutch has had it and needs replacing, make sure when you take it in that you mention the bearing needs to be replaced. Some garages do it anyway, but just to be on the safe side.
Hope this helps.
cheers
Vince