u can check two things
1) disconnect your AVR and remove, take multi meter select V~ (minimum reading for accuracy),take the reading of brushes and two poles, reading has to be between 3v to 7v if it is lower on brushes, take out the brushes physically inspect brushes for any possible wear n tear now install it back, if the value is the greater on brushes means ur rotor is faulty and if it is higher or lower on poles means ur winding has a problem. if both are with in range then if is probably AVR which is faulty.
2) u can get a cut on ur rotor copper bearing area by lathe machine, with heavy usage those plates can get deteriorated with the passage of time and create problem.
i hope this will guide you......
forgot to mention one important thing, generator will continue to run during reading taking procedure ........