You may have to slap it harder ! Still look for cold joints on the
hoz drive transformer. After that the hoz out & flyback area.
Cold joints on the H drive are common to most brands for many years.
when working on ANY set from the mid 70's on always resolder them.
If they keep up you could blow the HOT & power supply.
Your problem is very subtle so if its not a PC con it will be very hard to smoke out. You may have to wait for it to get worse or fails first.....
Good luck
73 Zeno
LFOD !