Oscilloscope scrolling
Trigger circuit works OK @ 10 MHz but doesn't quite work at lower frequencies.

This kind of fault is very often due to degraded electrolytic capacitors in such old equipment. Many fixes have been accomplished with a scattergun approach: replace every electrolytic capacitor in the trigger signal chain.

Or you can start with C501, C502, C503, C508 in Unit 3 PCB where trigger signals are processed. Probably safer to replace any/all you see.