Thanks for tips, both.. Re temp:

Fortunately there is: smcFanControl, which I installed days after acquisition of the iMAc.

And I always attend to Temp--in any electronics--I have WInter/Summer values for the little fans.
iStat gives me The Word every time I look, esp. at GPU, often warmer than CPU.
(Once plugged in an ext. mon. It worked, but I don't use, so removed) Not related to--

As to that video glitch thing--I upped a couple fans (none is video-card specific) a few hundred rpm; last event was ~October IIRC.
No plans to r&r that card after first securing New Tape and other watchmaker fiddles to disembowel the world's largest laptop. Not soon anyway, I hope.

This machine has been nothing but a treat, after the progression from PDP-8 --> Osborne1/Otrona --> and all the Beastware greasy-kid stuff + the daily, Will-It-Work?
If it dies irremediably next year, will just look for another 2yo and move from 20"-->21.5" BIG-screen!!
(People who never owned Beastware before getting an Apple.. just Can't appreciate their Apple properly, I wot.)

You've been there since PDP, a bit different perspective.

My PWW thought I was nutz for taking a new laptop, reformatting it, tossing win 8, installed Win 7 and required apps, and handed it to the wife for school work.

He thought I gave it a lobotomy by "downgrading" it.

I tracked down "In the beginning there was the command line", threw it on the projector screen, and told him to read it.

THAT's 60 pages or so, I'm not reading that!

FU, you are are reading the 1st couple of pages so you have a clue of what an OS is, and then we can have the discussion.

5 pages later he had a good start, and I emailed it to him to read the rest in his room. I'll quiz him.

He started with Win, is a minor gamer, and does minor windows bitching but it is his world.

Welcome to IWETHEY!