I'm with Willy on the voltage suggestion. Try bumping it up a little and see if it will run stable at a higher speed. But, WATCH YOUR TEMPS. Upping the voltage really causes temperature rises.
I have the Duron in my sig running 750 @ 1035. But, my mobo did not have voltage adjustments or multiplier adjustments so I am a little limited, but I just closed the L7 bridges on the chip to give me 1.85 by default and it purrs right along.

Only problem is my Volcano 6Cu will only keep the load temp down around 57C, which is stable, but way hotter than I'd like. I'm experimenting with ducting to see if I can lower it.