1996 Windstar with 3.8 and 101,000 miles. Starts fine and idles fine in park or neutral. When I put it in drive or reverse it will either stall or the idle will surge between about 500 rpm and 900 rpm. This also happens when slowing and stopping for a light. I changed the AIS with new Motorcraft and sprayed the MAF with cleaner but this has not solved the problem. Air filter is good. What do I check next? Thanks.

 

Sounds like a gasket leak around the manafold !