Gonna need a bit more info.
First, what do you have the cpu voltage set at?
Second, what mb are you using?
Heatsink/fan?
Depending on what you have the voltage set at, if you are still low enough, you might bump it up to the next setting. You don't mention if your board is a S370 or a slot 1, with the chip mounted in a slocket. The latter usually offers more voltage adjustability (depending on the slocket used).
Its also quite possible that you have hit the max for that particular chip. I have a 600 cC0 that will boot into Windows @ 1008, but locks up almost immediately. It locks after about three hours worth of 990 MHz, but is rock solid at 945. (1008/990 at 2.05 Vcore, 945 at 1.9v)
I'd like a bit more from it, but it just can't cut it. 945 is still one heck of an overclock from a 600. 994 is great for a 566, the majority don't get that high. Congrats, you got a good one.
Post some more specs on your rig and we can help a bit more.
Mr B