i would have your windstar checked for a blown head gasket, you have all the signs.
they can check the van with a chemical reaction tester, that basically checks for exhaust blowing up into the coolant filler bottle.
this is the cheapest way to check for a blown head gasket, which these vans are notorious for, we see an average of 2 windstars a week here for blown head gaskets or worse "engine replacement" due to the coolant wiping the lower end bearings out.
head gasket replacement with parts runs around $2200 and around $4500 for and engine replacement. those prices include parts and labor based on an $87 per hour labor rate.