[sdiy] 555 osc lock-up???

[peter edwards]

I'm building a circuit where two 555 oscillators are feeding to two  
inputs of a NAND network. When ever the frequencies get very close,  
the two oscillators lock together!
Anyone know how to fix this??? I can use something other than a 555  
if it would work better. ideally the circuit would be relatively  
simple. The output needs to be close to square (50% duty cycle)  
across the audio range of frequencies. I've been experimenting with  
the 40106 but it had the same issues.
Thanks!
-pete