Have become so accustomed to, (It Just Works--as promised..) only rarely do I look in on how well the crankshaft and pistons are doing.

Query for the mondo-informed, if you please?

Have had, now 3 obv. video-related crashes, in last 4-6 weeks:
vertical off-white bars, with 'corduroy like' striations, maybe <1/32" every ~1/8" of white expanse.
An earlier crash may have had a slightly different appearance. Dead to inputs, ergo--Manual shutdown. Restored all, correctly, each time.

This would seem to be Nature's Way of easing me into the conclusion: something is going bonkers in the GPU? or other graphics-related. Note also: via smcFan Control, GPU is kept under 130ish °F, just on general principles--and because smc is so neat a watchdog. That is usually hottest monitored locale.

Are there any graphics test suites for iMac? Though, as a seeming intermittent/likely hardware (?) problem, that might reveal nothing, unless say, a stress-test could cause the effect.

Nada in Crash Reporter or User Diagnostic Reports.

Any obvious next steps? Meanwhile shall whistle past the Apple Guts cemetery.
(3.75 years since getting a .75 yo machine is.. not quite as long as I'd 'expected', but wtf; that's Random fer ya.)

Gracias