As the title says, booting will occasionally give me an error saying something along the lines of "radeon: ring 0 test failed" followed by "radeon: disabling GPU acceleration" and then freeze right there.
I can't manually reproduce it, as it seems to happen randomly. When it does happen, I generally force it off by holding the power button, because the few times I've tried to let it work itself out, it stayed there for a couple hours. Sometimes it only happens once, other times it will continue saying the same thing for hours on end.
Once it stops happening, it will usually be fine for a couple of days, and then come back as soon as I start to forget about it.
Google has only turned up vague bug reports for other distributions that didn't even have a definitive answer. They repeatedly ended with the OP saying "nevermind."
I've been using Ubuntu for years and never had this problem. It only showed up when I made the switch to 14.04. Also, I didn't use the upgrade button. It's a clean install.
I'm running 64-bit on an HP Probook 4530s with hybrid graphics.
P.S. I know I said I've been using it for years, but there is still a lot I don't know. Even if you think it should be obvious, stick to baby-steps, just in case it's something I haven't run into before. My knowledge only goes as far as what I've needed to fix in the past, which isn't much.
Thanks for your time.