I've had the (2) best transmission places in the area and my long time Euro shop test drive the vehicle and they all said the trans and shifter seemed fine but several days after I get it back it will lock in 6th gear, they all claim they can't diagnose the issue until it locks in gear with them. I've been told its the clutch, the trans, a leak in the input shaft, and a worn synchronizer plate. If I replace the trans and still have the problem then it's the clutch or vice versa. In my opinion, there are two options:
(1) Replace the trans & clutch for about $5500 (2) Sell the vehicle for 1/2 it's value so the new owner's purchase price + repairs will equal the current car value.