Ok, think you're looking in the wrong area.
My guess is that it is the solenoid relay, and these are my reasons why:
The relay is hot switched, and the contacts get pitted and wear out over time.
Now, on a motor, the only thing likely to wear are the brushes, but the relay gets more abuse
If the relay contacts are pitted, they will have high resistance. So, the thruster may start to run, but then the contacts heat up, causing loss of drive current.
So, there are ways to test this theory.
Firstly, locate the thruster relay, and smell it. Do this very intensely. If it's pitted, you'll likely know.
Second, you can get bypass the relay with heavy duty jumper cables, or custom made wire jumpers (heavy guage!!!)
Try all this, and see what happens