I would put money on it that the problem is low mains voltage. During periods of high demand, the voltage of the mains can drop up to 10% and this is enough to prevent the voltage regulators in the VC3 operating properly. During the design stage it’s always a compromise between overheating of the regulators or possible hum problems…. if you live in an area where the mains voltage fluctuates, then this can be a common problem!
If I’m wrong, please let me know and I will fix your VC3 with pleasure, but I suspect that this is the problem! 