In Sydney,Australia, the local power authority injects mains signalling for off peak control. This is called 'zellweiger' I think which I have never actually seen but have had to cope with the effects of. This particular system is not synchronised with the mains so when it drifts into the zero cross time, it causes the detected zero cross to wander significantly. Even something like X10 could upset your system.Also, hash from electric motors, light dimming etc may cause problems with your circuit. In my circuits I have some basic RC filtering and some heavy filtering in the software by using a phase & frequency locked loop to cope with mains borne interference. Maybe the above could be the source of your problems.