Put the top in service position and disconnect the drive cables from the motor under the clamshell in the rear center. You can then have someone watch and listen while you engage the switch to see if the drive motor is running intermittently. If it is then it may be a microswitch, or the motor itself. If not, it may be mechanical (stripped gear in the transmission?)
See the 101 projects site for details on the parts
http://www.pelicanparts.com/techarticles/Boxster_Tech/71-BODY-Convertible_Top_Repair/71-BODY-Convertible_Top_Repair.htm