Hi Zoojar,

Do take following pills each one at a time,

1. put a 0.33 uF / 300 volts cap at the switching contacts of the relay. I had exactly the same problem when I made a washing machine timer.
Each time the motor switched it washed my code and not the clothes.............

But if you are using a non-inductive load to switch this may not help because this may not be the reason.

2. Have you connected the pin 9 of ULN2003 to +ive 12 volts (the rail for the relay ??) I think this will help, not sure.

3. Follow the advice by Rauf, use a 100 uF cap at the output of 5 and 12 volts regulators.

4. Try removing ground coil and put this coil in series with the a.c. of your supply.

5. Most importantly, make the mains source for your micro supply and for the load, "poles apart". I have experienced this helps.

6. keep the load far away from your application, so that the noise decays considerably before reaching your micro.

If none of above pills worked, let me know I will change the prescription.

Get well soon, wish you a good health.

Simon