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Getting a bit frustrated here. I have one particular (used) Slot.it 23k motor that's got me puzzled. 

Put it cold straight onto the track and the car couldn't pull the skin off a rice pudding. It'll pootle round at one (slow) speed for the whole heat.

If I run the car with the back wheels off the track for 15 seconds or so before the race the motor turns into a real screamer, outperforming most of my other cars (magless.)

I'm reluctant to give up on it as when it performs after a no-load warmup it is awesome BUT - I can guarantee it will give up the ghost if it's rested for a minute or so. In fact it's really embarrassing if the start countdown is even a little delayed as it's back to being a dog again and will barely drag itself off the line.

Should I write it off and just swap in another motor, or is there a solution for this behaviour?

[rallyhub]

Maybe oil in the motor? 

[Kevan]

You'd get smoke coming out if it was oil on the brushes...maybe the brushes are worn to the nub and there's very little spring pressure

[StuBeeDoo]

Maybe oil in the motor? 

That ^^^ would be my first thought.  

You could try taking it off the chassis and running it in a bath of either meths or isopropanol.

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Thanks. That's given me a couple of options.  

Try a motor clean first, bin it if it doesn't work. 

[Steve A]

Have you tried placing the car facing backwards on the track, hold wheels off the ground and rev it .
Running the motor backwards can clear dirt off the commutator, saves taking the motor out of the car, so maybe worth trying first?

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Ah, an even easier option. I'll give that a go first.