car stutter

hi I just bought a 94 galant from a private seller who did not disclosed what was wrong with the car, drove the car home and noticed that after 10mins of driving the car started to stutter and lose power I basically drove it home on clutch, it cant even climb hills it only happen when it gets to temperature, i was thinking of the maf sensor I checked the spark plugs and replaced it, same thing, checked the fuel filter, checked the ignition/coil by unplugging spark plugs one at time and seem working fine, K&N drop in filter is right in the path of a hot radiator fan not sure if the IATs defective so I hooked an ohm meter and heat it up see it goes up and down when its cool, cant think of anything more, please help....btw the previous owner not anwering his cell anymore, might send him black ninjas and take care of my aggravation lol.