Problems like this tend to be catastrophic rather than intermittent.

The simpler suggestions are always the best ones to start with. Don’t go changing motors & gears until you’ve ruled out dirty track/wheels/pickups.

The Wheel/Track interface was my first thought, too. How often do you clean your track & Locomotive Wheels? What about where the pickups contact the wheels? If you are happy with those aspects, try the loco on your track with a borrowed controller and try a borrowed loco on your track with your own controller. If the intermittent problem persists, try your loco on someone else’s railway. What you need to do is scientifically establish where the problem is without being invasive; don’t break out your screwdrivers & soldering iron until you’re pretty sure where the problem actually lies. As soon as you start interfering, you run the risk of introducing new problems, making diagnosis of the original problem more difficult.

I had a Bachman 2 truck Climax with stripped gears. I sent it to Bachman for repairs. They called me and said that the 2 truck model was no longer available, but for $10.00 they would send me a 3 truck Climax. Bachman is very good about that, give them a try.

Low voltage equipment is particularly susceptible to ‘cold’ solder joints, ones which were insufficiently heated, or were moved while being soldered. These can sometimes be identified by careful inspection of the ‘color’ of the joint. A shiny, smooth ‘flow’ of solder, tapered, is generally good. A grayish, matte finish should be suspect! A clean tip, hot soldering pencil applied with a small dab of flux can firm these up, with a swab with alcohol to clean. otherwise, look for a break in a circuit board, or wire. As you say, the track probably isn’t the fault.

here is the picture of my train after I rebuilt it.