Perhaps a PAT test would the best way to determine what the problem could be. I assume we all get all of our equipment PAT tested every 6 months :
No, sadly not. A PAT test can be done by any Tom, Dick, or Harry, (fif you don't beleive that then search the net)., but it helps if the tester has some form of formal PAT training.
The tester does not in any way have to have any kind of electrical knowledge or backround.
Even if they did, the test would only establish a pass or a fail, and further more would not neccesarily pick up an intermittent fault.
PAT Testing is like an MOT on a car; it proves that everything was functioning (or not) on the day it was tested. There is no legal requirement for it, but its a good way of proving you took reasonable steps to ensure the safety of your employees. However, its worth mentioning that whilst there is no obligation to PAT test per se, if it's in your H&S Policy then you MUST adhere to the re-test dates which you’ve set yourself.