You must have some amount of rythmic irregularity in the brake disc to pads contact, but, the 126 chassis is known for problems around sixty. The common cause is weakness of the longitudinal strut mount (sometimes called rear control arm ball joint - basically the rear attachment of the lower control arm part# 126 330 13 35).
The discs should be dial indicated for run-out and probably sanded to remove glaze. You should also sand or replace the pads. The purpose here is that friction coefficient differences point to point around the circumferance of the disc can cause a pulsing force even on true discs. Both BMW and Volvo have factory bulletins out about this condition on their models that also have weak longitudinal supports.
Actually upon thinking about it, if it was in my shop I would throw away those discs. I have never seen a turned rotor that I liked. There is only 1mm (0.040") wear tolerance on each face of a new rotor so even one cut gets you very close. Also most manufacturers now recommend not turning rotors because the surface finish roughness is too great on all field service equiptment. MBs brake discs are designed to be tossed about every second set of pads.
Owner 24 bay BSC
Bosch Master, ASE master L1
26 years MB technician