boot problems

Hello,
I have a computer that is having trouble seeing the IDE devices when
booting. I can go into the computer and unplug and replug the ide cables
and the machine will boot normally and run for an hour or 2. The machine
will then lock up, and upon rebooting the ide devices are not recognized. I
had the same problem about a year ago and bought a new motherboard. It
fixed the problem for the past year and has now returned. Is the
motherboard the only thing that could cause this to occur or are there other
possibilities that I should be looking at.
thanks
bill