Make sure nothing is binding.
The fact you said the car runs well and then slows down,
I do not think its your brushes. Could be a weak brush spring, when hot it goes soft.
The car should warm up to operating temp and run all day. That is not what you described, yours runs warms up and stops.
You have a heat issue.
Like rich said, low power out put from wall warts won't let you burn it up to damage.
Put that car on a big amp supply you will be able to bake pizza.
Look at the pinion, with my mega, I saw the pinion more or less held the arm for spacing. Make sure there is no friction in this area.
Bad gears can make heat
soft pick up tension, whilst the tension needs to be stiff because neo tractions are used with this car.
What comes with the mega is all over the place mostly being to stiff but I wouldn't rule out being soft.
Check all your friction points. Check them when cold check them hot. Are they the same?
Whilst parts are interchangeable, they are not the best.
If given a choice I will always use what is made for that car.
In the case of hop up parts in the kit, well that is a waste of money.
With brushes and springs, using anything other than stock parts is a major improvement but, it changes things.
Everyone will tell .006 springs are the replacement, it works. Yeah but is it the same?
I think mega springs are made from all the late 70 steel china was selling to US auto makers, wasn't so good then ain't no better now.
With putting quality brushes and springs are what is needed, you may have to much tension on the comm
Use a smaller brush length to relive some of the tension.
When you spin the wheels by hand it should continue to spin a bit, to free wheel.
If you spin the wheels and it stops right away, to much tension and it will make heat not speed.