Bear in mind that the electric outlet bank may be sharing the current with other high energy usage devices (aka centrifuges, refrigerators, cryostats, etc) especially devices that intermittently draw power (aka refrigerators) will create this periodic issue. Review your electronics training on apparent power and inrush currents.
Additionally, the size of the wire (in the wall) supplying the circuit can be a culprit. Loose outlet screws bonding the wire in the outletetc... I have seen similar isses that were traced back to poor electrical work. Do not asume they did what was written on the blueprint. I had one facilty that detroyed a single phase laser due to poor wiring; a $100,000 "mistake".
Let me know what you find.