Sometimes you must monitor your memory timing speeds, often they change automatically depending on how much you overclock your FSB.
For example:
2.5-4-4-7 might become 3-4-4-9 after overclocking.
Ultimately making your computer slower. You can either attempt to manually set your memory timings (only if you have good quality memory would I do this), or find a nice balance between FSB overclock and memory timings increase.