Many utilities have a loading gizmo that they can temporarily attach at a meter socket to flush out problems in their equipment and cables. It’s primary use is to show open neutrals, but it can also show other “hot-lead” problems.
On the customer’s side, sometimes rapping a rubber-handled screwdriver on electrical boxes containing splices or terminations may expose the problem. The trouble spot may show up as a noticeable temperature rise with a “back-of-the-knuckles test.”
Intermittent electrical problems can be hardest to locate. If you’ve spent a lot of time on one day snooping, tell the customer that you need to come back later when you’ve had a chance to thoroughly visualize, conceptually map and mentally explore all of the many components in their electrical system. ;-) Maybe the place was once a haunted disco, or needs some new Feng Shui.
Was the house 15+ years old with an ITE/Gould panel? If so check the buss connection with the breaker and I'll bet some are burnt. It doesn't have to be ITE, but that's what I've seen most. I've also seen the same symptoms with a bad main breaker and a bad meter base.
Do you have an infrared thermometer? If so this can easily find loose connections because they will be much hotter than others.
New house, less than 5 years. All Squared D QO series. The distribuiton panel is Square D, NQOD. All connections/terminations seemed fine. Its a very large home with a 600 amp service, parrallel 350s, all copper, CT metering.
It possible, you may try temporarily putting much larger lamps in the fixtures to see if that affects the pulsing complaint. Clearly they could not be left in the fixture—only in your immediate presence during the test.
Sometimes the only solution is a methodical process of elimination.
From what the owners say it happens at least once or twice a day. The pulsing I witnessed lasted about 20 to 30 seconds. Not super rapid, maybe two pulses per second at the most, but defintetly a very even pattern.
I had a similar situation a couple of years ago. It turned out that the well pressure tank was water-logged and the pump was kicking on/off several times in a row causing the lights to pulse. If they are having well problems, I'd look here first.