Hopefully, you are using a diode across your relay coil to snuff out ringing, and capacitors on your power connections to your timer. Since you're getting triggers from external circuits, you might need to shield your circuit form externam EM, and isolate the trigger pin from the rest of the circuit. A suitable value resistor connected in series with the trigger pin might do the trick, and perhaps a small capacitor connected between the pin and ground to pass transients. Also, there is usually a capacitor connected between unsued pins on the timer and ground. I don't remember right now which pins normally requre this, but you can easily find this information using the web.
Good luck.