G4212B waves in baseline

We have seen some "waves" in the baseline of several G4212B detectors. We thought it was due to the power line, but we have taken many measures to avoid it (UPS, filters, mesh wires, etc.). Additionally we verified the same problem in another laboratory of the company (located in another place)
The curious thing is that they have an almost perfect sinusoidal shape, with a period of approximately 7 minutes.
I attach images of the chromatograms (even one corresponds to the reading at 360 nm, which rules out a solvent/pumping problem)

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