MP 1,1-5,1 Mac Pro 3.1 - display goes black and then fans ramp up

[iRacer]

Hello

I have a MacPro 3.1 - early 2008 that has the following issue.
It used to be a random issue but now it pretty much happens all of the time and it can happen at any stage.
Basically the display will go black and then the fans ramp up and the computer is frozen. I have to hold down the power button to shut the Mac Pro down.
Sometimes it happens on start up - about half through the startup bar, the display blinks, then goes black and the fans ramp up.
Other times, I will be working on it okay, the display will go black and the fans ramp up.
There have also been times where the display will become distorted with diagonal stripes across the screen and the Mac then becomes un-responsive.

Specs:
Mac Pro 3.1
2 x 2.8GHz Quad core Intel Xeon
OS 10.11.6
Ram - 6GB
Video - ATI Radeon HD2600XT 256mb

I have a Samsung Evo 850 250GB SSD for the boot drive and a Western Digital 1TB 7200rpm HDD for storage.
Connected to a 20" ACD

Does anyone know what could be wrong?
I'm thinking its a hardware issue - maybe the graphics card?
I haven't tried a different card, as I don't have access to one, and at this stage I don't want to beuy another in case it could be a software issue.

Thanks

 

[Macschrauber]

I bet for the GPU.

As a TEST you can remove the heat sink of the gpu, heat the gpu processor up shortly to 300 celsius with a heat gun / hot air station.

If it runs well after that replace the gpu. Heating up changes the contacting of the bga balls but not cures anything. Will come back soon.

 

[iRacer]

** Update **
I ended up purchasing a used ATI Radeon HD 5770 from a mid 2010 Mac Pro.
The install was straight forward. But when I started the Mac back up I got a black screen and screaming fans pretty much straight away.
Tried a a few different configurations with ram, drives and graphics cards. I even put the old card back in to see if the computer would run but that card only bought up a screen of artefacts.
Tried a PRAM reset, no joy.
Then I did a safe boot - bingo - the Mac started up and recognised the new HD5770 and also the old HD2600.
I put everything back in and removed the old HD2600, and everything was working perfectly.
I decided to add a second display and the black screen dramas happened again.
Another Safe Boot, new display recognised and we are back in business.

I have been using the Mac for a couple of hours, surfing the web and watching YouTube. No issues!!

I didn't try baking the old card, as my luck I would have cooked it.