Hi
3027 is rather an early 1960's model.
It is equipped with a Telex coupler.
The inversion relay does not only reverse the loco, it also activates the telex.
On top of the relay is a little cam activated contact.
Run the loco without the body.
When you reach that "speed limit", you're likely to see little sparks on that little breaker contact.
All you have to do is to slightly increase the tension of the little spiral spring that pulls the relay's plunger back.
I say slightly and progressively as you have to find the "sweet spot": the plunger doesn't move unless you actually send the overvoltage inversion pulse on the transformer, and at the same time stays put while the loco is in its normal running speed range.
You should also carefully clean the contact in question, but be **very careful**, it is very delicate , and once you "bend it wrong", it's very hard to get it "right" again!
Good luck.
Cheers