hi, Nair,,

I would like to share some experiences with you regarding this problem.
once i have comeup with the same in industrial envoirenment, my MCU bourd was going mad when something comes to change the state in the nearby electrical zone, I also used SMPS to power the board.

I was keen to get rid of this. since I read about the ground /earth lekage in SMPS circuit.
because of SPMS running at very high frequencies and probaply housed in metal frames, they are much shielded than other to do thier task. but certaily if the MAINS power line get some distrubance , the ground line of the DC terminals are connected to the MAINS earth line via low ESR/ high voltage capacitors and cuase minor (some times very large spikes)in the DC output rails. this happens usually in old SPMS.

having said this, you may get my point that, check your MAINS earth line secured properly and better to replace the SMPS with new one.
I guess the fualt not in your MCU board, but in the SMPS and MAINS ground/earth line.

wish, this will help you..

thanks...