Restarts also can be caused (I didn't see someone mentioned that before)
by bad sectors on the harddrive(s). Driver problems also can be connected to bad sectors (problems with reading files on HDD, for example).
You can try a WD diag utility from the manufacturer,
but... none of manufacturer's utilities says exactly "how good" or "how bad" the drive is. They are saying only - "passed" or "failed", but drive could be between these two opposite states... I've seen drives that "passed" manuafacturer's tests but nevertheless were clearly dying (clicking, freezing and so on).
I prefer latest MHDD utility (v4.6 supports serial ATA drives)
or similar utility called Victoria
http://www.benchmarkhq.ru/english.html?/be_hdd.html
or HDAT2 utility from http://www.hdat2.com/