Check the brushplate as well. These rarely wear out on an early 675/2025, but if the brushes and springs don't fix it, that would be my next guess.
The early 675/2025's are the best running steam engines of the postwar period in my experience. I've got 4 of them, and they're my most often run engines on the postwar layout. I'm in the process changing the 2055 motor to a 2025 motor. Should be done in the next 20 years if progress continues at the rate it's going now.
