Something that has been bugging me for a while on the Mercedes is reverse gear. I tend to reverse off my drive, having parked nose-in to the garage and quite frequently, while slipping the clutch the car will jump out of gear.

When I got the car it did this in reverse and 1st sometimes. I changed the gearbox oil for some MTL Redline stuff a while back, and following advice on various MB forums, put in ~200ml less oil than came out as this was said to improve gear selection. It did indeed, with 1st having not jumped out of gear since, however reverse has been getting steadily worse to the point that it will jump out of gear roughly 50% of the time now unless I gently hold the gearstick in position.

Googling the issue returns virtually nothing, because my Mercedes is a six-speed manual gearbox, not auto - which means its one of about 3% of the total production run!

If it was a FWD I'd have a serious look at the gear linkages, but since its RWD I'm less sure on this - essentially the gearstick is mounted to the top of the gearbox so there are no external selector rods? The clutch seems fine, no slipping or anything - by is hydraulic so there is no (?) adjustment on that either I don't think?

Living with this for a while, I've found that if it jumps out of gear, and I depress the clutch and put it back into reverse there is a much more 'positive' engagement - like the gear cog engages fully. I don't get the same when I'm selecting reverse from neutral. Its almost like there isn't enough throw on the gear lever to get the gear to fully engage first time round. I've tried selecting a forward gear first, then into reverse but this doesn't seem to help. 

When it jumps out, it tends to product a loud thump and usually a grinding noise - which I suspect isn't great for the longevity of the gearbox!

I took the centre console apart to get access to the bottom of the gearstick

Which looked a bit grubby and nasty to me. Above it it in 3rd gear, below is in reverse

The balljoint seemed to all be caked with oily muck, but incidentally, with the gaiter, rubber concertina thing and foam sound deadening pad out, the gear selection is really good, far more positive than before. I put it all back together making sure to tuck the rubber concertina thing well out the way of where the gear lever goes when selecting reverse (left of 2nd gear) and it seems better - I wasn't able to make it jump out of gear moving the car about on the drive. Maybe the concertina thing had got snagged between the gearstick and the reverse lockout thing, preventing it from selecting properly? 

Concertina thing for reference:

It looks like the ball joint and bushing are not available separately, and you need to replace the whole base and stick arrangement with part A203 260 44 09 - and this is a cool $700.

I'll keep an eye on it, fingers crossed it was just the rubber concertina boot thing snagging up and preventing reverse being engaged fully.