I'm pretty sure I know what you problem is. I ran into this very issue when I tried folding on my 450. It turns out, the most stable drivers that work with the 450 is 266.58. Try downgrading your drivers to that see how it goes.
The latest 273's work for every other card except for the 450, so I'm kinda bummed about it. Also, the v7 version was very finicky detecting my GPU's (All 450's) so I reverted back to v6 using "FAH GPU Tracker v2". Really made a huge difference - all my cards are folding right now like a champ.