Damn! We are living with this bug since 4.4.1 update which was launched in november… And blizzard do not fix that bug yet coz they connot reproduce that(what a shame). Sad times for all players who get this bug.
I was mange to found this topic – [Cata Classic Bug]: FPS drops after 4.4.1 update
Autor of this topic was attach some logs with error – https://gist.github.com/tav0x222/ad8e03b7b6226252d9a9fe0c920eacbe and https://gist.github.com/tav0x222/046e63bd059a801bb3eb2a0caf0101ba.
Maybe devs will check out EU forum and this logs will help them to reporduce this bug.