Not to sound like a broken record, but the above recommendations are all spot on. I had your exact problem several years ago, ended up pulling the transmission and sending it to Gbox for a full rebuild. My advice as others;
-Change transmission fluid with Porsche OEM, pretty easy job.
-Gbox 2nd Gear Fix / insert.
If those 2 attempts don't produce results you may try altering your shifting as others have suggested. If still unhappy it may be a good time to pull tranny and deal with the IMS and put in a new clutch like I eventually did, Good Luck.......
My write-up from a few years back:
IMS, RMS, Tranny R & R Tips - 986 Series (Boxster, Boxster S) - RennTech.org Forums