I think players like that are by nature pretty inconsistent. You're guy reminds me of one of mine:
http://www.whatifsports.com/HBD/Pages/Popups/PlayerProfile.aspx?pid=5344361
Both have high power, but not elite splits and average eye. And coincidentally both play in Buffalo. My guy didnt have as big of a dropoff, but the season where he dropped from 54 to 35 homers was pretty frustrating.
I agree with the comment above that he was probably overachieving during the 59 homer season, and that you should probably just be happy you got that from him. High power with a low eye leads to an inconsisten player, unless the other numbers are high.