Like someone had mentioned on here it could be plugs or wires or even a vacume leak I have a 2001 ranger that did that for a bit and it had a vacume leak, there is a good way to tell if it is a vacume leak however it can mess up an O2 scensor doing this. open the hood with the truck running and spray a chemical called ChemTool on the motor if you do this and the truck all the sudden revs up when you spray it on one part of the motor then that can narrow down where you vacume leak is at least in that general area. If it is not a vacume leak and you spray it on your motor it will just make it smoke for a few minutes till it burns it off but it wont rev up. Once again this can ruin an O2 censor but if it is a bad O2 scensor it will cause it to idle rough too. best of luck!! | 
