Chrysler had a similar problem back in the 80's with the L-body. 3rd gear would pop into neutral while driving, especially over bumps. The fix according to design engineers was to strengthen the detent holding 3rd gear in the shifter and increased the draft angle 2° at the synchroniser teeth to increase the axial hold force under load at the 3rd gear.
I don't think this info will help solve your problem, but I thought this precedence might be interesting. I was not only an owner of an L-body with this problem, but one of the engineers handed me the new and improved parts for the fix, and explained the problem!
95 E320 Cabriolet, 131K