There's a rubber bushing on the shift rod. Once it deteriorates and ultimately disintegrates, gear selection is iffy.

My 190E 2.6 five-speed has three of them - same part as the one used on autos- that I replaced a few years ago. The one-two rod bushing at the trans lever was completely gone, which made first gear difficult to select, and the others were deteriorated and about ready to to disintegrate despite only 82K miles. Time and heat take a toll.

Pelican has them for a couple of bucks each.

Duke