I assume you remove the key from ignition when not in use? These cars have a lot of trouble with the ignition switch, so I would look at that as a possible culprit. The starter is not on a fuse as far as I know...I wonder if that could be pulling juice, but not enough to spin the motor?
Also post on
www.986forum.com and
www.renntech.org if you haven't already. Those boards are very active compared to this one.