I think that any credentialed Porsche mechanic should have the software (Durametric is minimum) to tell you if you have any of the major mechanical failures you're describing.
He should be checking Cam deviation, Cam timing, fuel trim etc...
Does it run fine until it is under load?
There are so many things that can cause the vague condition that you've described. Things like a soft fuel pump (won't supply adequate fuel under load), bad ignition wires, bad coils and more. If the car is not throwing a CEL, I doubt you have a serious timing issue.
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