Stress test makes core temps jump 40 Degrees. what's wrong?

You're right. 15-20C is about right between idle and load. Some really overclocked/poorly cooled setups will run a 25C difference, but a difference of 40ish is what your indicating. Sounds like exactly what happened in my tests when I mistakenly knocked the heat sink loose. Check your little plastic pins, if the heat sink is on properly, the pins will not be loose in any way. Oh, and NEVER let your CPU go past about 75C, or you risk making a paperweight out of that CPU. my 6550 runs at 400FSB, 32-35C idle, and never passes 65 on the hottest of days, and I'm using stock cooling. There is something very wrong with your cooling setup.

Yah -I'd believe a loose CPU cooler. It's obviously not working hard at idle, so the temps aren't too out of control. But when you push it, boy...

No matter what the problem is, I would suggest dropping down your OC while you try and find out what the issue is. Or, like BCBoy said, you'll have a paperweight in the end. Once you've located the problem, start overclocking again.