Next suggestion would be to boost the vcore a bit on your cpu to see if it helps resolve the problem.

Sajin: Interesting...
 
Question: Why would I need to increase the vcore voltage now, after many years (since 2007) of it working well, at standard voltage until recently?
 
Would the CPU have developed increased resistance due to aging over the years?
 
Oh, and how much of a boost?

Cpu is aging. Needs a bit more juice to make it run stable I bet. 0.10 should be good.

My Asus PC Probe II is reporting Vcore at 1.27v
3.3 at 3.25v
5 at 5.02v
12 at 12.21v
CPU at 46C - to 50C at idle (4 webpages open)
MB at 43C
CPU Fan at 1950 RPM
 

You many want to try 0.05 bumps instead then.

Interesting...
 
I was just about to write down my Vcore voltages and noticed that it is fluctuating from 1.14 to 1.27
Computer was idling with not input from me.
 

Let me know if increasing the vcore helps. 

Well, I bumped up the vcore voltage a wee touch... but for some strange reason I'm still getting the same readings on PC Probe II.
Not quite sure what to think about that - it's been quite a while since I've fiddled in the BIOS, and it was a bit of relearning, so maybe I have to go back and check it again.
Since last posting, I have updated the video driver with a fresh install - same as the previous updates, no help, see photo).
I'm not sure if this is related, but it just feels like the computer is running slower than it was before going into the BIOS. I wonder if I under clocked it  ;-)
 

 
I get this about a dozen times a day!

One step at a time. Let me know if increasing the vcore has resolved the blue screen issue.

He should try to increase CPU voltage more...
But you see all components are old, after some time it's normal to problems start.
Maybe is GPU, maybe is 10-12 years old PSU problem, maybe 8-9 years old motherboard and CPU?
I know they could serve more, but today Internal Graphic on i3 HD530 work better I think. GT620 that's Fermi architecture?