My Mac Pro boots but the screen shows pixels and squares

Is this the grahics card?

I noticed a few weeks ago that my mac pro powered itself off, i didnt really think anything of it at the time as it was in sleep mode as i was away from it for a few hours.

Because i know systems collect dust over time each year i clean them out using either compressed air cans or a mini compressor to blow all the dust out, because it powered off i thought it maybe a build up of dust so i cleaned it out.

Then took it in and booted it up and lal appeared to be ok.

Then about a week ago i booted it up and then the screen went all pixelated and in square sections.

I checked everything is seated correctly and removed the ram, hard drives, graphics card and reseted them, some times it boots ok and then goes to these screens, other times it sits at these on boot.

Any ideas or help would be most appreciated.

Thanks

Trev