as the van is driven the idle speed gets higher , turn off the ignition and it starts off normal until you step on the accel.

it sounds like you may need to clean your throttle body or check for a vacuum leak

sounds like you have a bad throttle body sensor,check to make sure it is working properly and replace if bad. good luck

check for a sticking cable on it also some of those the cables would get a bad spot in them and cause that to happen,yours may be a sensor going bad on it from the way you described it doing,you can have auto zone or any parts store scan the computer and tell you if its a bad sensor causing this too happen,and its free,good luck.