Running rough, but a little perplexed

[87pathy5spd]

So I few weeks ago I started getting the hesitation/bogging like it has a bad cat. Some power loss but not too bad. I filled her up the other day but used mid grade instead of the regular stuff. It greatly decreased the hesitation and bogging. Could this be something besides the cat? I rarely have the car over 3500 RPMS. It seems to be effected mostly between 2500 and 3000 rmp. I assumed I didn't have significant power loss because I belive the head studs or whatever are cracked, haven't looked but sounds like a big exhaust leak from the engine bay.