Far too many possibilities to analyze from here. I'm doubtful about the clutch. Could be ... blown head gasket, warped head, cracked block, timing mis-set, etc. etc. etc.
Quickie analysis: Pull the spark plugs one at a time and inspect them for signs of fouling. I assume you've already determined the engine is not missing. Check the oil's condition, and if it isn't there on the stick or looks thick, filthy, etc. you've found part of the problem. If all the plugs look bad, replace them and see if it helps.
This car needs to go to the shop, and I mean a Porsche Dealership.
I have owned many of them, and I would look at your tranny mounts, and your rear end mounts first, then if they are all OK, look at all bolts in your suspension, something is lose, and if it was a internal problem, you would have other symptoms.