If big gearless motor....
Check to see if the interpole are rocking and shorting out against a motor field connection. Very similar to what you're talking about as far as results. Only seen it once in 30 some odd years, but I wont forget it.

It Don't mean a thing if it aint got that swing.

Cool, Free, Johnny Smith courtesy of NPR..HERE
http://www.npr.org/ramfiles/asc/asc25.smith.asx