U 122 is the signal amplifier. The entire circuit is in there. The AT 127 is part of attenuation circuit. when you use smaller signals does this change the trace? Have you tried the adjustments on the attenuator (not the other adjusts)? Together they attenuate (or just not amplify) the signal.
The AT 127 would come into play with every signal coming into channel two. It is probably blown, it is the first thing a high power signal would get to. It looks like a propriatary part.
Unfortunately your flickering could be anywhere, It is hard to measure the extremely high voltage of the CRT for ripple (13.3 KV), need a high volt probe.
They also just list the HV rectifying circuit as another "box" , maybe one of the extremely high volt diodes inside is blown.. Would account for the steady flicker that would get smaller with slower horizontal trace.
Unfortunately, I do not know where to get these parts. Maybe someone else more familiar with this model of scope could help.