The beam is multiplexed to generate the readout so at some sweep repetition rates when the readout is free running, the multiplexing becomes visible in the trace and this is completely normal.
There is a switch on the readout board which controls when the readout multiplexing takes place. In GATE TRIG'D mode which is suitable for photography, the readout is generated at the end of the sweep but in this mode, there is no readout unless the sweep is present. Some of the other 7000 mainframes have this control on the front panel.