mine was doing the same thing a while back, i replaced the IAC valve and it seemed to fix it but then it started back again a few weeks later. i took the valve back and got another one and it didnt fix it. i finally traced it down to a vac leak and i havent had any problems out of it for a while

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2000 ranger 2wd 3.5" Fabtech lift, 31"x10.5" BFG all terrains, leather bucket seats, poweraid TBS, k&n, flowmaster, 2.5L... but soon to be 302