From your description of the test results obtained with the reversing unit bypassed the most likely cause of intermittent running is the brush assembly and brush springs. The lever style springs need to apply strong pressure on the brush holders and be carefully aligned. This was a marginal design and is the first item to suspect after the reverse unit is properly working.
I would not mess with the field centering at this point. If it is slightly off the motor might run slow but until the brushes, arms and springs are all correct it is premature to change the field centering.
