won't start

96 ford windstar van, 192,000 miles. I started my van and let idle with the heat/defrost on high for about 30 min, during the recent ice storm. I drove to my first place, shut off the van, went in came out about 7 min later. Van started fine. Next place same amount of time came back out and van started fine. I had turned the heat/defrost to low before leaving home. Last stop I shut off the van talked for a few min. then tried to start it and it wouldn't start. I tried several more times giving several min in between each try and only a few times would it start but then dies before putting it in drive. The whole driving from my house to the last stop was about 20 min. total. I thought maybe from idling for so long that the fuel shut off button had been tripped, but it hadn't. Also on my key ring there are 2 buttons. One for the locking system the other for the engine. I pushed it but the van still will not start. The van was running fine before all this. The engine cranks over very slowly. Did a volt drop check at the battery for the starter and it came back that the starter was alright. Now it won't crank over at all. The battery tests alright...it's a fairly new battery.