[6GH8cowboy]

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...