Normally I would suggest cleaning the wheels and the track first, before anything else. These things are always important in assisting good, reliable running.
However, what is obvious from your videos is that the power is getting into the locomotive, because the lights are coming on even though there is no movement. That means the motor or gear train are sticking. Now, I don't know the Mantua mechanism details at all, but in general, freeing up these things involves a bit of dismantling and cleaning any grease or dried oil off, then relubricating with a small amount of suitable light oil. Also, lightly oil the axles in their bearings.
Depending on the motor type, the brushes may need attention as well. There is little you can do with an enclosed can motor, but if it is an open frame motor you may be able to see whether the brushes are worn or not bedded in, depending on how much running this locomotive has had.
As I said, I don't know the Mantua set up at all, so my suggestions are, of necessity, rather generic.
I recently bought a second-hand Pennsy GG1 electric, made, I think, by Mehano for an American firm. It had motors and flywheels at both ends, but on test I found one end was seized solid. That didn't worry me too much as I wanted it mainly for display, but I had a look at it anyway, and ended up dismantling the gearbox and gear tower completely at the seized end. What I found was that the original oil had turned into a very solid glue, and the owrm shaft at the top would not turn at all. I soaked all the gears and shafts in Isopropyl Alcohol, before reassembling it and oiling it. I decided to clean and oil the mechanism at the other end as well, then ran it again on DC, and it ran very sweetly indeed. Not bad for my $AUS40 purchase.
Same as Southern Man suggests, and especially that the lights staying on in run 3 confirms that power collection is adequate.
I would therefore give priority to checking the connections from where the two wires to the motor brushes are connected to the board, the connections to the brush terminals on the motor, and the state of the brushes (if visible). It only needs a poor connection that breaks intermittently to cause the hesitations you are seeing in run 3.
(The hesitations and light flickering in runs 1 and 2 are typical of a mechanism that hasn't run for a long time, power collection unreliable until the moving contacts in the circuit begin to polish up against each other and conduct better. Possibly also mechanically a bit gummed up with dried lubricant in the drive trains, which has been pretty much sorted by run 3. I don't hear any mechanical distress when the loco pauses in run 3, which suggests that the problem is loss of power to the motor.)
There's likely going to be more improvement possible once you have reliable running.
The picture of the motor bogie is helpful. That has way too much lubricant on it in my opinion, some of it will move onto the commutator, brushes, pick ups and wheels over time and the running will degrade. My (cautious*) suggestion would be to dab most of it off with non fluffing cloth, such as cotton.
*You might want to wait for other opinions, not having ever looked at a Mantua mechanism this may be normal and the lubrication what's required for longevity and/or quiet running.
Good to know that you got help and it runs reliably now. Enjoy!
Hello guys, i found its root cause.
Recently the issue occur again. It's both hand contacted with motor is not stable as you can see in the video. I adjust it to touch a motor a bit looser. Let's see how far it can run smoothly.
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