Yes, Richard. All those things are worth checking. But my money's still on the voltage fluctuation. It wouldn't be too difficult to clip on a meter and see what is actually happening.

As some of you may know, I 'do' sound at a number of festivals, and this problem occurs from time to time. I remember one particular occasion,in a field of course!, when you could definitely hear the fluctuation. And when everybody decided to use the showers at the same time (heated by electricity of course), there was a massive voltage drop. A lot of modern digital equipment doesn't like this at all, and one band 'lost' all their keyboard settings when the voltage droped down to 183 volts. (For our American readers, it should be 230/240 volts in the UK, and was consistently hovering around the 200/210 mark at this gig).
The local electrician wouldn't believe it until we strapped a meter across the supply and produced the evidence.
It is caused by a combination of excessive load, and too long supply cabling (usually running from an adjacent building). This is why a lot of festivals now use generators, which, I don't care what anybody says, are considerably more stable and consistent.
Graham