Some thoughts on that problem.
1. The shifter mechanism is worn.
2. The shifter mechanism has 2 cables. One is the gear selector (blue cable), the other is the shift cable (black cable). The lenght of the cables has to be adjusted correctly and the cables should be free. It's difficult to adjust the cables correct without the Porsche special tool (9619).
3. The 2.5 Litre 5 gear transmission has an adjutable linkage between the two cables on the otside of the transmission. If this linkage isn't adjusted correctly you have a higher shiftig friction in general, or 1nd, 2nd, 5th and reverse cannot be enganged, or cannot be engaged easily. To adjust the linkage you'll also need a special tool.
4. The later 6 speed transmssions don't have that linkage. But there is also a need that selector and shifter work perfectly togehter.
5. The shifter mechanims reacts if cables are not lined correctly. Some mm tolerance can lead to good or bad. So also a bad rear engine mount can lead to a bad shifting mechanism.
I would say number 4 is your problem. Maybe in combination with another problem. So if you think of 3 and translate it to / combine it with 4 i would say this is an adjustment problem and check that first. But the adjustment will be complicated without the special tool.
Added 2 photos.
5 gear transmission with linkage:
6 gear transmission without external linkage:
