Like RVR said, RAM can fail but a more common cause of failure is some kind of corruption. Sometimes corruption can be fixed.
If memtest is detecting errors, it is likely a bad stick (not fixable), but not always. I'd have to see it and play with it to tell you for sure.
Dust can cause ram to act corrupt. It may help to blow out the RAM slots, etc. Not likely to help, but I once encountered a PC that had been in a house fire and they had cleaned the outside up really good, but the inside was caked with soot. Of course, it blue screened. I would too. I'm told that's a fairly common thing (dust messing with the connectors). My PCs are pretty dusty and never had problems
