I would say it may have something to do with your O2 sensors. That reads your exhaust and says if it need more or less fuel and adjusts the injectors. You may also want to have your injectors proffesionally cleaned. Could also be fuel pump is starting to go. I am listing these assuming you checked the air filter and throttle badge intake making sure there is no debris or other obstructions. All the other things I listed are a pain to do yourself and are better off taking it somewhere, unless you have the proper tools. O2 sensors for example will probably need some serious heat to break free as well as a special socket.
Complete rebuild axle/engine/everything swap in progress:
Parts already gathered for assembly: 2001 Ford 302, 1990 Mustang GT 5.0 ECU and wiring, NP435, NP205, Pair of axles from 04' F350 SuperDuty (Sterling 10.5 Posi rear, Dana 60HP Front-Both Disk Brakes 3.73 Gears), 37" H1 Tires, 9" lift TJ Coils for 2 link conversion in front, Stage 2 Clutch Kit, 3 Row Aluminum Radiator, Chevy Astro Steering box (4" front Stretch), XJ Rear Leaf Springs (6" Rear Stretch), and lots more.