Dear Hendryawan,

1. In your circuit you are using RC reset. Switch over to some good power supervisor with power on reset. You can use MAX810, MAX812, DS1232, MAX701 etc.

2. Make sure that the 100nF decoupling capactior is as close as possible to you micro Vcc Pin.

3. If you are switching an AC load through the Relay than do remember to use a good snubber across the load. And if its a DC inductive load use a fast switching diode across the load.

4. Get rid of each and every ground loop in your circuit.

5. Do remember to read the noise related articles in FAQ section.

Though may not be required in your application but in my designs I always use following...

1. All outputs and inputs optically isolated.
2. A 10uF tantallum in parrallel with 100nF decoupling capacitor.
3. A Ferrite bead and a 2.2 Ohms resistor in series with micro power supply [ Vcc ] very close to micro.


Regards,
Prahlad Purohit