An unusually high number of hardware failures.

If got servers in a 4 post rack enclosure and for the last few years we have had nothing but bad luck with hardware failures. We've had 3 sas drives fail on a single server in the last 6 weeks. Not to mention countless other drive failures in other systems. We've also lost a raid controller and a processor on various systems in the last couple years.

The systems are on a UPS that is inspected annually. The enclosure has cool air pumped in with a blower unit and air filter in the bottom of the cabinet to eliminate heat and dust. We even started switching out Dell Servers for HP servers and everything seemed to go well for a bout 6 months until we lost the 3 drives recently. What else should I be looking at to get this figured out?

EDIT

Thanks all, for the advice. I'm going to have to find out if this rack is properly grounded and vibration is also a possibility.