MacPro 3,1 graphics card issue: suddenly very CONTRAST-Y?

I have a 'good 'ol' 2008 MacPro (22ram) I was running through a Dell 2408WFP via mini displayport. A few years ago I upgraded the stock card to the ATI Radeon HD 5770 1024 MB a couple years ago. And since I occasionally do graphics I calibrated everything, and had a good-looking screen for years. Never futzed with anything, because I never needed to. The machine runs quite well overall on El Cap, and it’s important that I keep it running for a few programs I need to stay alive (CS5, Aperture, etc).

But this past spring, I downloaded an El Cap security update, restarted, and BOOM: suddenly my monitor was very “contrasty.” Like the blacks are all being crushed and the highlights are totally blown out. I restarted, safe-booted, etc, etc, a few times. But still the same look. No other normal “graphic card failure” symptoms.

So I tried clicking through my pre-existing calibrations…but they were all kind of terrible. Not even really a visible change from one-to-the-next. Which is odd. I even tried doing an Apple advanced ”calibration" and it came out terrible too. In fact the “white apple” calibration (the last one) didn’t even work: I would move the slider and the entire desktop image would change, not the white apple. So then I tried tweaking out my monitor settings, and nothing helped there, either.

I tried to "repair" the calibrations, but nothing changed. They still seem to have little-to-no effect.

I did the PRAM thing, didn't help.

I plugged the monitor into my laptop and the monitor looked great again. So it's not the monitor.

But, since the Dell was getting long in the tooth (some burn lines, etc), I bought a new monitor, and something strange happened: my desktop picture changed on startup! Which is…weird. One of the issues I had w/ the Dell was that the main desktop picture kept reverting to the “Mavericks” wave image, not my custom photo. But now, with the new monitor, my custom photo is magically there again.

This makes my brain hurt, becasue how would a monitor determine the desktop picture? Anyway…Unfortunately the image quality is still the same with the new monitor: blown-out whites, crushed shadows.

At this point I’m assuming it’s a graphics card issue. QUESTION:

What can I do to troubleshoot this? I remember installing the ATI Radeon HD 5770 was a pain in the butt, bc it’s slightly too large for the bay.

Is there any way I can check my drivers? To make sure they’re not corrupted?