SDK's are nearly impossible to take back in time, so to speak. So if you had 2.1 installed, then installed the 12.3 drivers, it will have installed SDK 2.6 (the latest). When you uninstalled the SDK, or so it says, then when you tried to reinstall 2.1, it didn't actually reinstall OpenCl. I have spent about a billion hours with my mining rig trying to get it back to 2.1 from 2.5 and it's just not going to happen without a fresh install, and even then I get errors. So I said screw it, I'm mining with 11.7 and SDK 2.5.
Ps. I was on the wrong page, now that I see your speed issue, use CGminer to change the gpu speeds: Click anywhere on the CGminer window, enter "g," enter "c," enter "0," and then press "e." After that, key in your desired clock speed and press enter (if it prompts about the speed being high, just press y (for yes)).