It sounds to me like all the ill behavior of the transmission is due to the make shift(no pun intended) 190E shifter. If the right shifter were used with the transmission, it might be a dream to drive.

I don't think any of the ill behavior is because it is a turbo engine with a manual transmission. If I could get the right pieces gathered up, I'd like to build a car like this myself.

Have a great day,