Phil, courtesy of Nomake, I have only now checked your circuitry and have been negligent in not doing so earlier.
The open ended floating input will have a high impedance and it is little wonder that overload has been a problem.
C3 is included specifically to bypass high frequency noise not controlled by smoothing capacitor C1. “Capacitor C3 should be best physically connected directly between pins 16 and 15 on the IC, and C4 likewise 16 to 2.
I do not know how you have made up the circuit, but short current paths in these areas are important. “Sensitive” circuitry is involved and earth loops can present problems. It is important that all ground/earth points are physically close.
A dual IC is being used and I would suggest that the second unused input (pin 10), as a precaution should be grounded rather than be left open. However this is probably not that critical.
You have certainly set a pot boiling and I would suspect that you may by now have become frustrated as a result of the number of cooks invading your kitchen. I commiserate accordingly.
