I am working on an old PC that uses DDR2 800mhz, when testing the memory is memtest and memtest+ is sometimes gives errors (usually on test #6) but then sometimes passes with no errors. I have tried 2 different sets of RAM and have tested both of these sets in a laptop I own, both sets have never reported any errors in the laptop. Yet in the PC it sometimes does and sometimes doesn't. The PC is an Acer L3600, a small form factor desktop PC that uses laptop type RAM. The laptop can only run the memory at its max of 667mhz though. Sometimes 30 errors sometimes 20,000 and crashes memtest. I can't adjust the speed of the memory to try it at 667mhz, even though the board supports 800mhz.