Hi
My avr micro 8515 on/off a relay by a ULN2803 driver,when relay have no load everything goed well,but when I attached a 100w lamp as load to relay contacts the avr resets when turns the lamp OFF.
I used the snubber circuit across contacts but problem remains.
you might want to try a triac/scr type switch. Some of these come with opto isolator type inputs. They turn off by waiting until the ac line current reaches zero and then opening up. This way there are no severe line transients.
I don't really know AVR micros, but I share some of my experience with PIC.
I have a circuit with PIC16f876 that turns 30A (220 AC) relay; and even producing noise with a connection made with touching to wires of the load, every thing works well.
Did you add "Freewheeling" diode across the relay coil?
If your relay and your MCU are operated at different supply (such as relay at 12V, MCU at 5V), suggest to add some opto-isolator at the relay driver input.