explorer surging?

have a 96 ford explorer, 4.0 liter. I have only had this car for 3 months, no problems until about a week ago. At a low speed around 30 miles/2000 rmps it has a slight engine surge, at higher speeds it drives fine. When I take my foot off the gas pedal and coast the rpms fluctuate and it surges. I have taken it to the local ford service shop and they diagnosed it, but no codes came back. took it to another shop and he said the engine was misfiring but after having it his shop for an hour and a half he couldn't find what the problem was. so I took it to another shop who told me that the IAC valve (that I replaced with a aftermarket IAC the previous weekend) was the problem. So I repleaced it with a motorcraft IAC. I am still having the same problem. Does anyone have any idea's, I am going broke and I am quite frustrated with service shops right now.