Agree with Kat.....make sure coolant level in reservoir AND radiator have not dropped creating an air pocket....then check heater core hoses, that BOTH are hot.....if they are could be a temp door problem.....if one is much cooler, then try a reverse flush like Kat said....
Also, some of these vans had an auxiliary coolant pump....not sure if this one does......it is located under the cross brace in the engine compartment on the driver's side...