Could be a bad TPS but I would start by cleaning the throttle body and MAF. Also, when was the last tune up? It may be time for regular maintence.
'05 4.0: G-Force T-5, MGW Shifter; VMP MAF/ CAI/93oct N2O Tun; CHE K-Brace & Torque Limiters, Rr Ctrl Arm; Shorty Headers; Centerforce Pressure Plate, McLeod DF clutch, Adj Slave Cyl; 8.8" 4:10's w/ T-Lok; Coast Alum D/S; FRPP Dual Ex; 18" Bullitts; CF Hood; Rr Louvers; SS Gen 1 Frt Cover. ZEX N2O - 100 shot bests: 1/4mi ET 13.495/MPH 103.95 (*only mods were VMP N2O tune & CAI; shorties w/ duals; t-lok).