Just curious what all have you done to it maintenance wise? I'm guessing it either is a fuel,air,or spark problem. So i would say if you haven't done so start by checking your air filter,plugs,and plug wires. Mine is a 97 and when i got it, the thing was somehow running on 3 cylinders.
Idiot i bought it from sold it because of that mainly. Went to do the plugs and found that it wasn't firing on all cylinders and plug wires were shot. Three wires pulled out of the boots. So we done plugs and wires then had to drop an ignition coil in. Ever since never had a problem. So i would say if you ain't done it check your plugs,wires,and air filter. Also may be a clogged fuel filter. If you replace your plugs if you ain't already i strongly advise Motorcraft. I tired a set of autolites and i wanna say bosch. My truck hated both. Got maybe a year out of them if that. My mechanic talked me in to the motorcrafts i believe double platniums and it's been i think 2 years and still going strong. Hope that helps you out. Just throwing out what i learned from mine and man have i learned some crazy stuff.