No, you can't lock it. I'm assuming your using the 3:4 memory multipler currently, the only options are to switch the memory multipler to 1:1 to reduce your memory bus speed which of course will lower memory performance a bit or buy faster ram. If your only going to gain a few more Mhz by lowering the memory bus then I'd just stick with the most stable FSB speed you can run with the 3:4 memory multiplier active to keep the ram performance up.