Dirty component leads exposed to RF at high voltages can eventually
isolate themselves from their solder connections. This also happens
with gold-plated leads.

#5 looks like a cold joint to me, but it is hard to tell from the
photo; it could just be too much solder. The top one in #2 also looks
bad, but it looks like a filter cap from your description and you'd
probably know if it was bad.

Rosin left on the circuit board will eventually corrode exposed metal,
but it happens over a period of decades.

The easiest way to re-do these (should you decide to go that route) is
probably to just add a little liquid flux, reheat the joint, and then
clean up afterward with IPA or flux remover. Acetone works great, but
it can take off the conformal coating and silkscreens too.

I once worked on a monitor I knew had a cold joint (it was losing
deflection and beating on it made the problem appear and disappear).
The one that was bad was one I'd looked at half a dozen times. I only
found it by tapping around the board with a dowel.