Getting bsod when playing games in fullscreen

You say you have one card with 'some technical issue'. What technical issue is that and depending on what the issue is, yes it could cause that. Try taking the card with issues out and seeing if your problem continues.

Would you tell me specifically the make, model of your graphics cards and the make and model and size of your power supply please.

Have you physically pulled one card out of the machine and tried running it that way? It almost sounds like it is not getting enough power from the PSU.

If your cards are water cooled, I wouldn't suggest it unless you know what you are doing. With the temps you got running furmark, I doubt they are water cooled. 90C is entirely too hot. I have a 6950 in another rig and I can't make it go over 68C. To answer your question, yes the power supply can cause that. I'm not telling you that is the cause, only one possibility. But if your hardware, especially graphics cards or CPU get too hot, the computer will shut down to protect itself. If the power supply won't provide enough power for your components, it will crash. Power supplies can degrade over a couple of years and especially the cheaper brands. You also need to know how many amps it puts out and if it is single or multiple rails. As you said, it has run well for a few years, so obviously something has changed. We just need to figure out what it was that changed. If you would rather wait on pulling the cards and running them one at a time, there are several other tests we could do first, but they are rather time consuming. I would like you to get a CPU temp monitoring program and a GPU monitoring program and monitor both constantly as you use the computer. Most cards come with monitoring programs, but if you don't have one MSI Afterburner will. It is basically an overclocking utility, but is good at monitoring things. You can set a custom fan profile with it too, as well as get temp, fan and other on screen readings too. I like using Core Temp for CPU temps, or Real Temp is good too. All these programs are free, but as with many of the free programs, be sure to check for the add ons they try to download with them. You can opt out of downloading the add ons. You can also run memtest86+ https://www.sevenforums.com/tutorials/105647-ram-test-memtest86.html . I would like you to run it for 8 full passes all at one time or until you get an error. Even 1 error and you can stop the test. Each Pass is 10 tests, each pass tests a different thing and each test checks something different. So, it takes 8 full passes all at one time to completely test your ram. It will take 10-12 hours to run it, so it is best run overnight.