An individual awarded a fellowship under the provisions of this subpart shall continue to receive payments provided in section
1134b of this title only during such periods as the Secretary finds that such individual is maintaining satisfactory proficiency in, and devoting essentially full time to, study or research in the field in which such fellowship was awarded, in an institution of higher education, and is not engaging in gainful employment other than part-time employment by such institution in teaching, research, or similar activities, approved by the Secretary.