Dilantin - does anyone have intermittent wild swings in their blood counts?

Every 12 months or so, my dilantin level - which normally sits within the therapeutic range suddenly soars to toxic levels (24 - 25), for no apparent reason. After the most recurrence of this phenomenon, my neurologist dropped my Dilantin dosage from (400mg daily x 7) to(400mg daily x 4 + 300mg daily x 3). Within 3 weeks of the reduced dosage, my dilantin level had plunged to around 10! Needless to say, within a day or so, I had a doozy of a grand mal (in Missouri, that means a SIX month driving "suspension"!!).