Drove van to work twice and smelled slight coolant smell. Kept an eye on temp and ran normally. (9 mile drive). Drove van back home and coolant smell was a little stronger and saw some steam coming out from under the hood, not a lot, and temp was still normal. Went to go to the store 2 hours later and went .6 miles and stopped at red light and noticed van started running warmer than normal (just barely above 1/2 way mark). Took off and it stayed at this temp for about 3 miles, then went up to 3/4 on the temp. Made it to store, checked reserve tank and was normal. Went in store for over an hour. Started up van to go home and the temp went back up after leaving the parking lot. Then, I noticed there was no heat blowing from the vent....just cold air. The temp then started fluctuating from 3/4 to hot but mostly stayed at 3/4 until about 100 yds from my driveway, where it went all the way hot. When I shut it off, a LOT of steam came out from under the hood but I didn't hear any hissing or popping sounds or anything like that. I know I should have pulled over but it was later at night and out on a back road so there was no way I was stopping. I just need to know if the thermostat is stuck or the water pump went out or what. My husband will look at it when he gets home from work, but I'd like to give him some direction on where to look first. Any help would be greatly appreciated!!