Re: [260.19.12 regression] Clock changes don't work
Not a solution to fix the bug, you'll need a reply from the devs for that to happen, but a permanent workaround for the problem could be flashing your card's BIOS.
Did it with my GTX 470 and I'm glad I never have to worry about my clocks again. Want to lower it again? Quickly edit your BIOS values and reflash, it only takes a couple of minutes.