actually problem you describe is quite common. it is usually to be
traced to poor connection which might might be further eroded
by arcing and that's why you cannot reproduce it. this also happens
due thermal problems if chip overheats but in this case banging is
unlikely to help. i suggest resoldering all connections on and around
vertical osc. and cleaning contacts on the yoke connector.
vertical osc/driver is usually single chip so it should be easy to
replace. it might have gone by now after long strugle with arcing load.
power for vertical circut is normally derived from coil/tap on the high
voltage transformer used in the end stage of horizontal circuit.
i've seen such problems (loose connection or cracked PCB) near
the transformer. same suggestion as above - inspect for cracks,
check continuity and resolder.
make sure to give some time to capacitors to discharge before
getting inside tv. if you get it to work even briefly - great, this is
time to start some testing. normally in such case one would use
cold spray to freeze parts of the circuit. cold makes things shrink so
if the problem is not cured it will be easily found by applying directed
jet of cold gas from the spray can. be careful since this can only
yield results if performed on running equipment so don't go crazy,
don't make spils, don't toch much etc.
sometimes i had to use heatgun first to get to working condition
before using freezing spray to pinpoint problem.
again, all this thermal stress is not too good so use it carefuly, don't
blow heat on heatsinks (should be obvious...).
i would suggest hair dryer instead of real heatgun (much lower
chance of damaging something).