RTS 11. Correct lenses for use.

I'd also suspect the issue is mirror slip, given what you describe. I can offer a couple of quick fixes that will not be permanent, but they'll be enough to confirm what the issue is. There are angled corners on the mirror, so if the full length of those corners are past the edge of the metal plate it's attached to (by 2mm or 1/8 inch or so), it won't function correctly, so you can visually diagnose whether we're on the right track.

1. Pick someplace inside that's warm - but not in front of a direct heat source - and lay the camera on it's back with the mount ring facing up. Let it sit for 24-48 hours, and the mirror should slip back into a more normal position. If the issue goes away when you test it afterwards... it's definitely mirror slip, and as long as the camera is stored on it's back, it should stay put and not cause more problems.

2. With the camera on it's back again - the double-sided adhesive under the mirror can be heated up with no more than 30 seconds or so of mild heat from a hair drier, and it should slowly creep back into position. It will eventually drop down again if left standing up, so this isn't a permanent fix either. If you try this one, put some distance between the camera and drier, so you don't cause other heat issues to pop up.

If your just selling these, and not wanting to fix these to keep for yourself... I'd suggest that you just confirm the issue with fix #1, and inform any buyers that the camera(s) have the known issue of mirror slippage... and let them do the more risky permanent fixes, if they'd choose to. There are permanent fixes for this, but it can go wrong quickly too, and requires some finesse and experience to not ruin the mirror or the pivot plate - or change the reflection angle of the mirror and screw up focusing.

The reason for my comment on informing buyers is, that it gives them a way to measure the repair cost and/or the time, tools, materials, and expertise to do the job for themselves. I wasn't thinking about buyer deception in an auction or a for-sale listing, and only as a means for them to see true value and future cost. My apologies if it came off as anything else, as it wasn't my intent.

As for the 'on the back' fix... make a mental note of how far past the edge the mirror has slipped when you put it in position, to compare how it may move over time until you test it again.

There's also one more test scenario you can try. If a Contax camera has mirror slip, not every lens on it will exhibit the same issue under every focus distance. Some lenses have a rear guard piece that never moves, and if the mirror position is off on a camera, it will always interfere with a slipped mirror. Other lenses have a piece that moves as the lens is focused, and will only impede the mirror when it's at or near infinity focus. The 50 Planar (1.4 or 1,7) or the 135mm Sonnar have moving pieces. If the mirror does stick with them, and then 'drops' as the focus is changed, the camera certainly suffers from mirror slip alone.

There's only one other issue I've ever read about that has most of the same symptoms, but in this case, changing the focus won't allow the mirror to return to position. It will only right itself when you power off the camera - remove the lens - and power the camera back on without a lens in place. A web search for "Contax mirror slip" will lead you to lots of info on both scenarios.

As for getting cameras repaired... Unfortunately there's no authorized repair centers for either Yashica or Contax gear, but there are a few good repair shops that can do general repairs. One place I can offer in the US is Midwest Camera, who are reasonably priced with good turnaround times, and have a good reputation for quality work. midwestcamera.com/film.html For other countries, some of the other EU members here might be able to offer someone local or at least closer that you could mail in for repairs.

Good Morning Members.,
Well, Biggles & Lumiworx. I  left the RTS11 in a nice semi-warm area for two days, on it's back. This morning with bated breath I connected the 1st lens, a Planar 1,4/50. Wound on and hoped. No joy; Tried a Planar 2/135. No Joy.  Next a Yashica DSB 28mm 1:2.8, no good here either. I then tried a Yashica ML 50mm 1:2. Well this worked on every f stop , but not f16. I then tried a Yashica DSB 135mm 1:2.8. This worked on every f stop. So what conclusion would you make of it? I am of the mind to sell the RTS11 with true description and all the lenses separately. To try and have the RTS repaired may be a bit of a task, but maybe there is someone out there that can turn the RTS around. I mean, it is in a beautiful condition apart from what we all think is 'Mirror Slip'. I could make a post on my local facebook groups to see that maybe there is a camera buff out there that can put this right. If it can be repaired locally then I will purchase it for myself as I had my RTS11 stolen way back in 1993 in Romania. It had a motor wind and two beautiful lenses, actually the two Planars aforementioned. Well thank you for you help chaps. I do have a couple of other Contax lenses I have not tried yet. Had a piece of luck last night to make up for this disappointment with the sale of a Carl Zeiss Distagon T* 1;4/50. So the elderly friends will be pleased. As I am too. Best of luck to all. 
Best Regards
Michael