So I'm working on a RCA 721 the Table top model
I've checked components and most of them seem to be OK, I can't seem to find what is causing this symptom, I just got a lead on another set of the same model and want to get it working well again.
This one was used as a interactive history display and it always had this flickering, was on for about 6 hours a day for a month. now its back with me and I wanna get it back to 100%

This is what its doing
https://youtu.be/PGZaEqM6Cs8
https://youtu.be/ME_otfvRYLs

I have a partial 721 chassis if you need a part...…..

Don't know what test equipment you have, but a quick and dirty way to trouble shoot the signal chain would be to bypass points along the signal chain with a cap to ground. If the flickering goes away your looking upstream from there. A good start is the IF feed from the tuner. Being a set with a power transformer pulling a tube will be quicker. If the disturbance continues go stage by stage to the CRT of needed. This is of course after checking mechanically touchy spots first. Remember your best test equipment is between your ears...