What kind of brushes are you using. We used to have brushes made where Otis had theirs made and they made some changes and we had all kinds of problems. For one, pull the leads on the brushes to be sure they are attached inside properly. Is composition correct (Brush Grade)? Grade on Motor and Generator? Does Armatures have a glaze on Motor and Generator. If so and the grade has changed, the glaze may not be compatible. Motor Grade and Generator Grade are usually different as you probably know and are not interchangeable. Trying a different brand in both could help. Your problems sound like problems with brushes. The strength of interpoles in Generator affects compounding strength tremendously. On many Otis 71 & 82 Generators when I was at Otis, we changed the interpole shims from brass to steel to change the strength and create stability. The strength of interpoles is determined by doing a "Black Band Test" which is quite involved.