The usual CPS diagnosis is : "Does the rev counter needle bounce a little when you crank the engine? If it does bounce the CPS may be O.K. Definitive test is to measure the resistance of the CPS. The CPS resistance deteriorates as it nears the end of it's life JFP posted the ohms-spec.
http://rennlist.com/forums/996-forum/766125-lets-discuss-crank-position-sensors-for-a-moment.html