last year I had exactly the symptoms you have described. I was confident (and quietly hopeful) it wasn't a transmission issue so I took the console off to inspect the shifter and found the shifter had worn away badly in parts which made any movement literally impossible. The shifter was literally impossible to move.
Long story short - new shifter installed and all was back to normal. Plenty of options shifter wise too, OEM 986,996,997 standard and short shift plus aftermarket options. I went with 996 standard as it was all that was available at short notice (in Australia)
Good luck and keep us posted
