Sounds like you may have fused the brushes to the armature.
I would bin this motor as it may seem OK but it has cooked and will fail again if driven hard.
Brushless will give you all the power you require. I suggest you invest in a wattmeter to measure the current draw and make sure the wattage is both within the motor and ESC specs. You may need to drop a few prop sizes but the speed will be max with the correct motor loading.
Please let us know how you fare
Dave

It turns out that the 8.4v 600 graupner motor is very power hungry, and itvdoesnt like part throttle usage. The safety circuitry built into the Mtroniks esc cut in to save the motor and esc. But when I took the motor out & tried to run it just on a 7.2v battery, it wouldn't turn. So I gave it a spin by hand, and there was big Klunk, and the motor started to run perfectly again. All seems fine now. But i dont trust it. So I've invested in a brushless motor and matched esc which im in the process of fitting now. Thanks for all your help and postings. Cheers, Dave W 😊

I had this problem last year with my Aerokits Sea Commander also powered by a 600.
While sailing under full power it suddenly stopped. After drifting for a few minutes I tried again and it came back to life.

When I brought it back to shore the motor and ESC were very hot. As there appeared to be no flotsam in the lake and the prop shaft was able to be turned freely my only theory is that ESC overheated and shut down and when it cooled off it functioned again.😊

Boaty

What ESC were you using?
My experience with 600 motors is that they are power hungry, smell a lot and get very, very hot. I had two in a Slipway Trent (they advise the ECO version, no longer available) and used to be able to smell them from the shore!
Could be the ESC internals circuitry shut down and may start working again when cool.
Any weed in your pond will definitely overload them, which was my problem.
Too big a prop will also cause overheating. I had 40mm 3 blade brass props in the Trent.
It's possible that the motor is to blame if it has cooked the coils and caused an internal short. Try running direct from the battery with an ammeter in circuit.
I am assuming you have checked your propshaft for free rotation and no binding, locknuts can and do come adrift and can tighten up on the shaft, which may explain the slowing down you experienced.
Please keep us posted with your progress
Cheers
Dave