Q: Mac Pro keeps shutting down, CPU light a/b overtemp on

I purchased a pre-owned Apple Mac Pro 2,1 this past December.  I mainly use it for editing my photographs on Adobe PS, using RAW camera filter.  The unit never gave me any issues, until early last week.  Here are the specs:

|8-CORE 3.0Ghz (2x Quad-Core)

|32GB RAM

|250gb SSD

|GTX-680 4GB V-RAM

| Dual OS - El Capitan (primary) and Yosemite on two separate boot drives.

It all began early last week, with an error screen when I opened a CR2 file on Adobe PS.  I got this message...

"Camera Raw has temporarily disabled graphics processor acceleration due to error. Updating the driver of your graphics card may resolve the issue. Choose the "Use graphics Processor" option in the Camera Raw Prefer dialog to re-enable GPU support." 

Within 5 minutes, I when to the preferences and selected "use graphics processor", then continued my work.  Afterwards, The Mac suddenly shuts down without any warning  whatsoever! No messages, no sounds from the tower itself, the power light remained on.  I then turned the power off.  I waited for one minute, and turned it back on again.  At the password screen, it would shut down again.

I then let the machine rest for 30-45 minutes, removed any dust build up inside using canned air, and powered the unit back on to perform some minor troubleshooting; along with other "light tasks" to prevent a system overload..... It shut down within 35 minutes of turning the machine on.

Again, I unplugged everything (including the power cord) to reseat the two riser boards. I powered the Mac with the access panel open, to keep an eye out for anything unusual.  All fans are running smooth, no signs of any problems.  Until I updated the GPU driver (Nvidia Cuda), the machine shut down again within 10 minutes!  Frustrated, I opened the side panel to only discover, two red LED lights (CPU A/B) on the overtemp section of the logical board.  After the cooldown, I rebooted the MAC and ran a tempature monitor I installed.  I noticed that the Northbridge Heat Sink shot up fast when I was running Adobe PS, please refer to the picture I have attached.

I have no clue what to do next.  I am unsure if I need a replacement PSU, or something else entirely.  Any pointers, help, and/or advice.  I greatly appreciate it!

Matt