Quote from Lost-Ninja
Tried with/without Optifine; with/without Twilight Forest (was functional without all last night but started again this morning)*; haven't deleted the options.txt, but will give that a go too. I've tried three different texture packs (Sphax (x64), Soar (x64) & Default)
*mentioned in another thread as being a potential source of the issue.
Also having this problem. Have you made any progress finding the source of it?