My first three guesses....
1. Vacuum leak, like a cracked hose or one that popped off
2. Bad gas....siphon out the tank completely...burn it off in your lawn mower or use 3 gal. at a time in your daily driver...and fill the tank with premium
3. A spark plug wire fell off (or one plug fouled)

70 Roadrunner convt. street car 440+6, NOS, 4-spd, SS springs '96 Mustang GT convt. street car '04 4.6 SOHC, NOS, auto, lowered "Officer, that button is for short on-ramps"