Actually I know the cause, it's our power supply. I had a long argument with our supplier and they stuck to the usual mantra about the voltage being regulated by law etc.
Eventually I stuck a chart recorder on the supply and showed them that it rarely reached 220 volts and at peak dropped to just under 200 volts.
They changed the overhead incomer and it rose slightly but it's still under what it should be.
Mind you, as I pointed out earlier, they earthed our supply with 2.5mm wire to a stake and their latest bill to me is for just over £1200. And I'm on a token meter.
Roy.