i will throw my 1.5 pennies worth in here. keep in mind that if you have a circuit that is not grounded properly then the feedback from that circuit is sent throughout the neutral buss in the panel. incandescent lights are terrible sources of noise and an improperly grounded circuit containing those lights can cause a temporary lull in voltage and peak in interference. if you flip on the light and it isn't grounded properly then you get what i would losely describe as a partial brownout. i would check the circuits that are affecting the system for proper grounding and go to the panel and make sure that all the connections are tight, as was stated earlier a spark, or lose connection can cause some trouble with your system.

you ask that if it was the case of bad wiring then why did it stop when you changed out the reciever in one room. you may have said , but is it the same reciever or a different unit, if it is a different unit, then maybe the unit you are using now has a better buss sytem for filtration built into it. it really could be that simple ( example- the high end Denon and Marrantz reciever are very much alike but the Marrantz has a copper buss which is amazing at eliminating noice in the system).

just a little electrical theory troubleshooting, hope this helped

good luck
proaudio