3.1 89' Cutlass Supreme- Hesitation while accelerating

104k miles.. has a hesitation while accelerating past 37mph and around 53mph. Before/between/past those speeds it runs smooth. Hesitation is noticeably worse in cold weather(snow). So far I've gone through and replaced the plugs/plug wires/sending unit/gas pump/gas lines with no luck. Any ideas on where to look next would be appreciated.

*When I was pulling the old plugs- one had a black substance(oil?) filling the gap. The hesitation did seem to subside somewhat after a full tune up.