2003 Ford Windstar high idle

My wife and I have a 2003 Ford windstar we baught new. At times the engine idles very high (1500 to 2000 RPM ) and will pull the van down the road at 40 MPH with out your foot on the gas. You can cut the van off and it will stop for awile. We have taken it to the dealer several times and they could not find the problem. I have checked the idle stop screw and found it to be ok. Replaced the idle air bypass valve even though it seemed ok. The problem seems to be in the eltronics. When it idles high I can unplug the idle air bypass and the engine will idle down. The high idle seems to go along with the alternator screeming to keep the electrical system going. We have had problems with a dead battery. After jumping the van off everything checks ok. Alternater and battery check ok. This has been a ongoing problem since we drove it off the lot. Thank you in advance Dave.