[sdiy] 555 osc lock-up???

[Paul Perry]

Yes, using CMOS 555s will help, and separate power supplies.
But in the absence of separate supplies, it might be worth trying to
decouple the 555 circuits from the rest of the circuit and each other
by running the power to them via a 220 ohm resistor with a .1
ceramic and 10u electro bypass on the 555 side.

Also, the coupling may well be (probably is!) via a shared ground
return for the two 555 sections.. try to get a direct path back to the 
power supply ground for both of them.

paul perry Melbourne Australia