[jb92563]

I don't know much about Boxster specific transmissions but in general it sounds like the syncromesh mechanism in the transmission is worn out and preventing the gear changes.

A transmission rebuild may be required.

There is also a known issue that could occur on rare occasions, frequently when a short shifter has been installed that the linkages may have popped out of their restraints and are slipping when more pressure is applied, or broken plastic retainer.

Checking involves removing the center console around the shifter back to the armrest.

Its fairly easy to do yourself and wise to check the cheap possibilities before getting the Transmission rebuilt or replaced.

Maybe someone with specific tranny knowledge can chime in.