I had a situation similar to yours...appeared out of nowhere on this same computer.
Trial-and-error eventually led me to check my RAM, using Memtest. I did so, realizing I had recently changed modules (I have excess RAM modules).
Memtest pointed to the modules, all except one. As long as that one module was inserted, the system ran fine...because that module's speed was acceptable and anytime a user runs modules of varying speeds...the lower one is the standard speed used.
Bottom line: I didn't want to use that module as my baseline, so I went into the BIOS and clocked my RAM speed down. All of my modules were fine, it was my CPU clock which was creating the situation with the RAM modules. The moral of my story: Just because Memtest says that a module is "bad", it ain't necessarily the module which is the culprit or the solution.
Edit: All of the RAM modules running at the higher speed...were supposed to be compatible with the motherboard, according to the manual. But that did not take into account that the motherboard was may have been "overclocked" when the chip was inserted into the board.
Edited by hamluis, 12 October 2008 - 09:22 PM.