A ton of good suggestions in here.... it's probably already solved.. but
I know you mentioned temps. are fine. Don't forget that the temperature throughout your case is NOT the same in all locations. Many cases snuggle away the HDs in their bays smack in the middle of a stagnant air mass. If you do not have AT LEAST 1/2" clearance (top and bottom) of your drive it will warm up. Stacked HDs filling adjacent slots of a bay create hot hardware. HDs really don't like to overheat.
So add heat to the list of possibles:
Bad PSU and/or unchecked power input voltage fluctuations from your house
Dirt, Dirt, Dirt, Dirt.... (did I mention dirt?)
IDE cables shorted due to pinching, mishandling, etc.
HDs located in stagnant air (ie: hotspot) and overheating to failure
Rough handling
Thrashing (constant HD activity due to virtual memory utilization)
and last but not least.... God doesn't like you
-Sid