Terry,
You pin 12 is a floating antenna when the N.O. pushbutton is open.
A quick fix: R1 100k 12 ___ //----/\/\---+------\ \ 11 | 13 | >O----// C1 --- .--/___ / 1nF --- | | '------------- // ===
You could increase R1 if 100k isn't aggressive enough.
An alternate would be to connect your existing 10nF input cap to GND, and have your 100k pull-down and 1meg pull-up swap places. Then you'd have 10nF to GND to filter RFI,
1 meg keeping that 10nF discharged, and 100k to charge the 10nF rapidly when the pushbutton is pressed.John Larkin's MOSFET timer is decent too if you don't mind gutting and starting over.
Cheers, James Arthur