Check your bags through at origin. If -- and that is always a big if - your flight arrives into FRA on time, you deplane quickly (pick your seat prudently), and don't dilly dally in the airport connection should be fine.

Be sure to get your FRA-JFK boarding pass either online or form SK at JFK before you depart for FRA.

Also be sure on what your connection time is -- "75-90 mins" is quite a difference.