"Chassis Intruded! Fatal Error. System Halted."

Hello everyone, bit of a head-scratcher here for you all- any help/ideas would be appreciated.

I recently (6 months ago) built a new rig. Main component of interest for this problem would be the MoBo, a Asus M4A78T-E.

It worked fine for the first few weeks. Nothing to complain about, until I started playing a rather graphics intensive game. Then I would experience random system lock-ups. (I attributed it to the game, not the GPU, as other games seemed fine).

Then about a month in something odd happened, I got system lock-ups doing non-gaming activities such as Internet Browsing or Word Processing. It didn't happen VERY often maybe once every 2 days. (I know that seems often, but I was overly tolerant.)

Then something else happened around the same time. Sometimes during boot-ups, after the Bios Splash screen I would get the error "Chassis Intruded! Fatal Error. System Halted." I could get around the error by simply restarting, and the system would work fine afterwards.

Recently, the problem has gotten much more persistent, happening sometimes multiple times a day. The Chassis Intrusion error always follows the system lock-up upon the first restart.

Things I have tried thus far:

Reset the CMOS.
Reseated the Chassis Intrusion Jumper Pin.
Tried booting up without the Chassis Intrusion Jumper Pin. (Won't Even Boot, gives a constant Chassis Intrusion Error.)

Any ideas?

Thanks.