I bought a Pocketlogger for troubleshooting on a different car but it works on the Boxster too so I thought I'd share. It can read and clear codes but the really cool thing is that it can log operation of the car. You can watch and record timing, load, O2 sensor operation, speed and various other things. It plugs into the OBD2 port (you have to pull the connector out of the bracket to fit) and you have to pick up a PalmPilot, but you can get a cheap used one from the list of compatible units.
http://www.pocketlogger.com/index.php?pid=plobdii