The Collaborative International Dictionary of English v.0.48:

Persistently \Per*sist"ent*ly\, adv.
   In a persistent manner.
   [1913 Webster]

WordNet (r) 3.0 (2006):

persistently
    adv 1: in a persistent manner; "he was asking questions,
           unavoidable questions, persistently..."
    2: with persistence

Moby Thesaurus II by Grady Ward, 1.0:

96 Moby Thesaurus words for "persistently":
   and night, assiduously, bec et ongles, chronically, committedly,
   day after day, decidedly, decisively, determinedly, devotedly,
   diligently, doggedly, earnestly, enduringly, faithfully, firmly,
   fixedly, for an age, for life, for long, frequently, habitually,
   hammer and tongs, heart and soul, hour after hour,
   in all seriousness, in earnest, in good earnest, indefatigably,
   industriously, inflexibly, insistently, interminably, inveterately,
   like a bulldog, like a leech, long, long ago, long since, loyally,
   month after month, morning, noon, obstinately, patiently,
   perseveringly, persistingly, pertinaciously, ploddingly,
   protractedly, purposefully, recurringly, regularly, relentlessly,
   repetitively, resolutely, resolvedly, routinely, sedulously,
   seriously, since time began, sincerely, singlemindedly, sloggingly,
   staunchly, steadfastly, steadily, tenaciously, the livelong day,
   till doomsday, time immemorial, tirelessly, tooth and nail,
   unabatingly, undyingly, unendingly, unfalteringly, unflaggingly,
   unflinchingly, unintermittingly, uninterruptedly, unrelentingly,
   unremittingly, unrestingly, unsleepingly, unswervingly, untiringly,
   unwaveringly, unwearyingly, unyieldingly, wearilessly,
   wholeheartedly, with a will, with constancy, with total dedication,
   year after year