Hi
Sounds very much like your HC system has a fault int eh transformer. I say this because you state that its fine after a cold start but after a while the fault occurs. As the unit heats up the magnet in the transformer expands and this could cause a short which it causing the loss of sound and subsequent hiss etc.,. Has the unit been moved recently or moved whilst still warm to a cold location? the sudden change in temperature could have caused the transformer to crack.
Not something you should repair yourself unless qualified.
DAve
May 2007