Does sound like some clutch slippage. What is your pedal engagement like? Has it changed? Do you mostly drive on the highway or city? Depending on what type of driving you do it will play on how much life you get out of your clutch. If your clutch begins to slip when the pedal is not depressed, then chances are your clutch disc is worn, or the spring plates in your pressure plate have worn out. If you have determined the clutch is in deed your culprit we have some pretty beefy clutch kits here:
Porsche Boxster (1997-2004) - Clutch & Flywheel - Page 1. They have just about everything you'll need/want for the job while you're in there.
Have a lot of info about the clutch and how to replace it as well here:
Porsche Boxster Clutch Replacement - 986 / 987 (1997-08) - Pelican Parts Technical Article
Good luck.