The horn will be putting all sorts of voltage spikes onto your supply voltage. This is because most horns are a coil (electromagnet) and pair of contacts. Each time the horn diaphram is attracted to the electromagnet, it causes the contacts to open, the diaphram then returns to it's original position allowing the contacts to touch again. The magnetic field in the coil collapsing as the current is shut off causes a high voltave spike, it is this series of spikes that is causing the timer circuit to reset at odd intervals instead of the cycle rate you have selected by the component values you used.
You would need to filter these spikes out of the supply to the timer chip to stop this from happpening..... try putting an inductor in the + supply line, with a cap to neg on the 555 side of the coil. It could also be worthwhile putting a 17 volt zener accross the cap to help shunt any remaining spikes to 0v.
- ---- ciol ------|-------- 555 power input | | --- filter --- cap | (17 volt zener in | parralell if desired) 0v
The inductor in the supply line will try to keep the current into the filter cap constant, as inductors resist change in the current through them. The filter cap will smooth out the remaining voltage spike left on the supply.
Pip