fca:
So its the big relay(with no 230v power presence) just by switching does any electric noise dirty that hangs the arduino ? some times it doesn't hang but the lcd gets strange characters ...
Put a diode in parallel with the coil of the big relay, right at the relay terminals, cathode to the positive side, anode to the negative side. Type 1N400x (where x = any of 1 thru 7) will do.