91 Mits Mighty Max Stutter/Missing/Idle Drops

Wel team just looking for a place to start. I just replaced the head on this truck after buying it for cheap. Body is in great shape just trying to shore up whats left of the bugs since it sat for a few years. After the head was back on and replacing the distributor rotor it ran well for about a week. Now its missing when I’m trying to hold a speed and its worse the slower I’m going. Seems when I get above 30 mph its less noticeable. I replaced the O2 sensor and Plugs as well when I bought the truck since it had been over heated and sat for a while. Where should I start in diagnosing this problem.