@cameo_2007
Your AC power is first rectified into pulsating DC. Then after filteration you get a DC supply, then you use regulators. I'm talking of a linear regulated power supply, in this case.SMPS decision on it is beyound the scope of this topic.These units are suspectible for normal working for less than 10ms, if you cut off the main AC momentarily, there internal supply is capable to keep the circuit alive for shorter amount of time. Usually, these are the figures in ms, depending on the device. So I don't think such a minute disturbance glitch in AC would causes the fluctuation in DC output of the sensor.
A Varistor is a good idea, as given by FVM,but I have not seen protecting device connected across the AC relay,we use in DC only. AC is already pulsating, I don't think there's any need to put any component block, that would kill kick back voltage of relay.