97 Audi A4 TDi Gearbox?

I am dirving a 97 diesel a4 and the gearbox feels like it is hopping forward when changing gear when I first bought it in May 07. I assused the engine mounts were worn out and needed changing. But since October, when putting it into 4th gear theres a crunch so I tend to clutch and let the revs drop right down to eliminate the crunch or just not use 4th gear at all. I have since brought it to 2 mechanics who both had 2 different oppions. The first said there was a bolt missing that goes through the subframe into the gear box and the second said the engine mounts were a bit soft alrite but the synchro was worn out in the gear box, both mechanics work in Audi dealership. I have heard about the crunch in an A4 gearbox before but never heard of the actual problem. These gearboxes are hard come by second hand and are quite expensive. Please leave me any opinions.
The car has 155k on it.

An 11 year old car with galactic mileage. I'd move it on tbh. Get something newer and fresher.