Head Scratcher….flashing lights

Be patient with me because I am not a lighting person. I am the Jack of all trades Techical Director of the building. We have an ETC set up. About 15 years old I’m told. Dimmer racks, wall panels, and Element console. We are having issues with our lights randomly flickering. Conventional fixtures, LED fixtures, and house lights simultaneously. It’s not associated with a particular show file, it happens even when the light board is off.  I’ve changed out the DMX running from the wall port to the board for brand new DMX cables and that didn’t help anything. The dimmer racks are not registering that there is an error. We never know when this flicker will happen and cannot re-create it on demand. Sometimes we go months with no problems and other times it happens every day at least once or twice a day. We pulled in the college electrician and she said that there are no problems with the wiring and is stumped. Does anyone have any idea what the problem could be?