Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
per-stŭdĭōsus, a, um, adj., very desirous, very fond of any thing: musicorum, Cic. Tusc. 5, 22, 63: litterarum Graecarum, id. Sen. 1 fin.
Adv.: perstŭdĭōsē, very eagerly, very fondly: audire aliquem, Cic. Brut. 56, 207.