I would suggest that you start by testing the clutch a litle bit.
Shift into top gear at 30 mph and with the clutch pedal fully up, floor the throttle. A weak clutch will slip under this kind of load. It may not at 30 mhp, but certainly by 40 or 45 you will detect that the engine can be reved freely without touching the clutch and without adding any appreciable acceleration to the car.
If it passes this test, then in my mind there could be one of two problems. You may have a bit of oil or grease on the clutch disc that only slips a moment until things warm up. This isn't likely, but I have seen it. The other problem would be that you likely have some air in the hydraulic clutch linkage that improves almost immediately once it is exercises. If this is the cause, you should change the clutch fluid and carefully bleed the system. If you have not done so recently, you should also change the brake fluid. It is the same fluid and they both share the same resivoir. You will see a hose off the brake master cylinder that drops down to the clutch master.
You could also have a problem with the clutch master cylinder or slave cylinder that would require replacing them, but it is far more likely that you just need fresh fluid with a good bleed to make sure there is no air.
