The Smallest Minority on earth is the individual. Those who deny individual rights cannot claim to be defenders of minorities. - Ayn Rand
I have to express a little sympathy for service techs here. Years ago, when I repaired/maintained copiers for a living, I had a problem that always occurred while I was away, yet the machine performed perfectly every time I was there. After half a dozen service calls (and the customer being ready to put the machine in the dumpster), I had the problem occur while I was present and found that it was power related. The outlet that served the copier was on the same breaker as the company's break room, and in the afternoon people would come in and cook things in three microwave ovens that were all on the same breaker. I happened to be there during break time and watched power drop from 113VAC to 93VAC. Of course, within 15 minutes the problem had gone away.
The point of this post is not to apologize for your service techs, however. The point is to help make you aware that there are many external factors that can affect the operation of any complex electrical system. The more complete, even nitpicky, you can make your list of conditions when the problem occurs and when it goes away, the better your tech's chance of finding the common denominators of the problem, which is of course the core of the art of troubleshooting. Something as seemingly unrelated as your spouse using a blow dryer in a bathroom on the other side of the wall can theoretically be the key to the problem. I suggest that when the problem occurs you note (if you can) time, temperature, humidity, local weather conditions, day of the week, appliances in use close by, and anything else you can think of.
Obviously I cannot troubleshoot the problem from here, but the more bits of information you give your tech, the more likely he is to spot a pattern.