That's an overheating problem and it's caused by the locomotive's motor asking the powerpack for more current than it can safely deliver. The powerpack's main power transistor is overheating, and the safety circuit kicks in to lower the transistor's temperature...which it does by reducing the current output to the track...and the locomotive comes to a stop.
The overload/safety circuit is operating properly.
If this were a DIY built powerpack (throttle) you could--theoretically--add a larger heatsink to the transistor, but do NOT try that here. That's a sealed case for a reason, and you should leave it that way.
The root cause is a locomotive motor that's asking for more current than the powerpack can safely deliver, and that's what you need to address.
One common, but not commonly diagnosed reason is a geartrain full of hair, carpet threads, dirt, and general gunk. If the bottoms of the trucks are open, so that you can see the gears, and if the locomotive has just been thrown into a box, left on the carpet, etc, then that could be your culprit.
Two options there: 1) The best one is to take the trucks apart, clean the gears and gearbox out *carefully* and clean each gear. Clean out the rest too, using a toothbrush or something similar, and then put it back together again and see if that doesn't fix the issue.
Note: The plastic gears will be made of Delrin, which is a slippery, self-lubricating plastic. You don't need to lubricate it to have it run smoothly...although you CAN...
...which leads to option two: 2) Lubricate an already gunked up geartrain. It goes against all reason to try this, as gunk is possibly what's causing your issue, and lubricants can often just pick up more gunk--which is why there are those well known words of wisdom: "lubricate sparingly"--but
sometimes the opposite can work, provided you use the right sort of lubricant.
I'll suggest this one: https://www.homedepot.com/p/Super-L...ic-Grease-with-Syncolon-PTFE-21030/202932687?
This is a gel, and it won't dry out over the short term.
PTFE is teflon, so it's slippery stuff (!), and it can work where more conventional lubricants pick up gunk, but don't really do much else.
It ALSO has some useful electrical properties--it could really aid with carbon buildup (it works wonders on classic, open can electrical potentiometers when they are spiking), so if you "invest" in a tube @$6.67 (get the grey tube version, not the cheaper one), you can try it on the motor bearings or commutators too, if needed. But take it one step at a time.