Well if its only when a recording is happening or a second client is viewing then theres obviously something to do with that no? Have you looked in log to see if theres any decoding failure errors or anything with the card? They ussually show as PES FAIL in tsreader.log.. Maybe something else to look at..
I ask about the drivers because they definately make a difference, I tried several but the only ones in my NVIDIA folder say 285.6x and 290.53 so I think I dropped the Teslas..
Tried LAV with DXVA2 (native) and it's fine (still can't use for live tv but the issue with recordings while playing MKV is no longer a problem.)
Also noticed that the processor was dropping in speed so fixed that in the BIOS - however that didn't appear to be the cause as I tested CUVID again and same problem.