Suggest a Search for 2 tests
1. Tach needle bounce
2. Ohms at CPS connector.
https://www.youtube.com/watch?v=9bLHL06eNvo
BTW it is called(in Posche-speak) Crankshaft
Reference Sensor Part # 986-606-112-02(for mine)
No need to guess. At least it is an inexpensive (Bosch)and easy replacement.
Porsche Boxster Engine Sensor Replacement - 986 / 987 (1997-08) - Pelican Parts Technical Article
Pelican Parts