I find myself on a number of calls just like your describing here.

Usually i will listen to the customers as to the signs and symptoms . This in itself can run from an informative diagnostic history, to a twilight zone rerun, depending on the person and problem, we've all been there.

My next step is to assess if the situation is an isolated circuit, and or device, machine, etc. , or if it is a system failure.

Many of these intermittent ordeals will cease to occur the moment i arrive, so in lieu of monitoring equipment, i sometimes try to recreate and/or alter the scenario.

I have run an extention cord to the panel from suspect circuits in the past, and used my PVC pipe heater, a good steady and known resistive load, while metering the panel.

This sort of thing may weed out a bum circuit.

I would consider looking for a large draw at the doc's office, does he have an x-ray machine ? autoclaven? maybe there an elevator motor cycling ?

I went crazy at one place until i heard the owner say that things had screwd up right after a lightening storm, and by golly, it was the UPS itself that had gone bad.