BleepingComputer.com: Mysteries of memory testing

When testing with Memtest86+ how should a single error that doesn't repeat be regarded? This PC133 SDRAM is fairly old. (Yes SDRAM, not DDR. ) And the system used to be unstable under Windows XP when it was running at 133 Mhz so it was dropped to 100 Mhz. Then it was stable. But since I have re-installed Windows XP and am trying to get Linux working on the same system I decided to run memtest86 and see what I could learn about any problems with the RAM. After running most of an afternoon I rebooted to memtest86 and it found ONE single byte error on the first pass. Then I set the RAM speed back to 133 Mhz and it found NO errors on a retest??? Since then I've set it back to 100 Mhz and run memtest86 several times and it has not found the error again.

???