Hmmm, this is probably grasping at straws, but your mention earlier in the thread that you live "close to Brooklyn" prompts me to ask if you happen to live near JFK airport. A member recently mentioned in another thread that in the past he had sonic issues in his system that were caused by radar that was in use at a nearby airport. The problem went away when he moved to a different city.
Perhaps sensitivity to that or other kinds of interference will vary as a function of the warmup state of the equipment. Grasping at straws, as I say, but who knows?
Regards,
-- Al