The rivet vias connect copper traces from one side of a board to the other.
They are filled with solder so looks just like a huge solder blob until
you clear it out.
The solder, rivet and board traces work differently with temperature so the
connection eventually cracks and becomes intermittent or completely open.
You can heat up the solder again but the solder will inevitably end up with
trapped air bubbles inside the rivet and it will crack very soon again - if a connection
can be established at all.
The best way to cure this is to clear out the old solder, feed a piece
of copper wire through the now hollow rivet and solder it to a suitable
place close to the rivet on both sides of the board.
There are twenty or so of these rivets in a CDX and they are found on the servo
and decoder boards (two large separate boards that are mounted one on top of the other).
Most - if not all - are ground connections, going to different circuits.
A time-consuming job but something you will have to do only once (per machine, that is).
Replace the capacitors while you have the boards out. They will also be - if not bad,
then at least marginal by now.
A cap kit exists.
Martin