When a light switch is turned on or off then the overhead projector's lamp turns off. Tried powering the projector from a different plug circuit. Same problem.

I have checked all power connections in DB. Voltages in order. There are 4 plug circuits in house. Tried powering the projector from each circuit with the others off. Same problem.

It is also random light switches causing problem. It also happens when the microwave ovens door is opened and closed.

Could the slight arcing that occurs in light switches be the cause and if so, what's the cure ?

Any ideas ?

Derek