w111 manual slipping out of 4th gear

[Scutch]

Hello, I just bought a 1965 220 SE and after two hours on the highway at 75mph it has slipped out of 4th gear twice. It seems to occur during the normal vibration that happens when I take my foot off the gas and decelerate from in the 70s to 60 or 50mph.

The rest of car is in amazing condition. Minimal rust, great interior, many great things.

Otherwise shifting is normal, clutch feels good,.

Could it be the shifter bushings? Or is slipping like this more likely an internal transmission problem?

Could anyone offer possible prognosises?

Thanks!
Eddie