common issue. fresh transmission fluid (oem or redline) can help 90% of the time.
there are also the following:
California Motorsports - Porsche 996 transmission section (shift arrester 3/4 down the page)
2ND GEAR POP-OUT UPGRADE-GBOX Transmission
the latter you can do yourself, the former requires pulling the transmission apart (at which point getting a replacement transmission becomes a cost-effective alternative).