Dodge 2001 Ram

I am the original owner of a Dodge 2000 Ram cargo van and I am (the van is) experiencing sputtering/bogging of the engine at sporatic times always within 3 miles upon starting and driving the van. It runs like a new van as I have had just about every thing replaced in it. I can drive the van for 3 months without a problem than have it happen twice in one day. All mechanics say the van is fine and when they run diagnositcs on it there is no problem. I have left them keep the van for a few days and no problem showed up. Any suggestions?