Have had the above Fan in my Toilet for over 2 years. Every now and then the fan will, for no apparent reason, come on, run for a while then switch of. it can happen at any time, day or night and is quite irregular. Will not happen for some months then start again. Have checked for dust internally but keep it regularly vacuumed. It is switched to come on when light is turned on and neon indicates power to fan. When it is running intermittently no indication that power is on. Help