Have you checked that your mirror has not slipped from the plate it is attached on? This is not uncommon with Contax SLRs. Your symptoms sound very familiar. CZ lenses typically have more protruding back than Yashicas, that's why the issues appear earlier when using them. When the problem happens try to slowly release the lens. If mirror comes down I am pretty sure that mirror has slipped due to glueing giving up. Investigate that mirror does not extend over the plate it is attached on. In case it does, google for Contax mirror slip, info is available.
-m-
