I think I know the clunk that you are describing. Sometimes this clunk can happen when you start to move the shift lever ever so slightly to the next gear before the clutch has actually disengaged the power output to the transmission. So Dean's advice of bleeding the clutch is a good place to start.
You'll need a long length of clear hose and a little bit of patience getting the bleeding set up properly. I like to put a box end 8mm (or is it 7mm? Can't remember) on the bleeder screw first, then stick the hose on there. Don't open the bleed screw too much or you'll have brake fluid running down the side of the trans. Bleed screw leaks at the threads quite easily if you loosen the screw too much. Finally, a pressure bleeder is a very smart move for bleeding the clutch system. Manual bleeding doesn't work well. I've done it and it just doesn't work well on the 911.
BTW, the shift coupler on the G50 tail shaft is different than the 915 trans. There's pretty much zero adjustment back there. The coupler is a pair of flanges riveted together with a rubber pad in between acting as a damper. No adjusting allowable with rivets.
Do what Keith said and check out the shifter assembly. There's a hard rubber/plastic ball cup in there that wears out.
It's #10 in the picture. #2 sometimes wears out too. There was an article in Excellence, maybe a year ago, that detailed how to service the G50 shifter. I'll try to scan it when I get a chance.