I had the same problem but for a different reason. I have sirens tied into a Gamewell municipal telegraph system. Occasionally we will get an open circuit due to a line down or box failure. As long as the circuit stays open the timers restart resulting in continuous cycling. I used a P B time delay relay set for 3 plus minutes (slightly longer than timer cycle). Problem solved.
PS. SIRENMAN did you get my message about brushes and springs ?