Most vintage color tv's have either a triple or quad
section selenium diode pack installed on the
convergence board for peak detection and waveshaping.
I think the problem you are seeing with the blue
lines sagging at both sides is caused by this diode
either being open or shorted or leaky.I can't
remember which failure mode it is but it's one
of them.These diodes develop the same problems
that the horiz.afc diode unit does.I think you
could use regular 1N4148 or 1N4007 silicon
diodes in place of the multisection selenium
to see if it fixes the problem.Otherwise you
could have an open control which has been
mentioned in the previous posts.Those controls
are wirewound and are relatively fragile especially
if they have not been operated in decades.
Regards,
Swanson