Ford Truck Enthusiasts Forums - Van won't run at lunchtime

I know this sounds weird but I have a problem. It has just started getting cold here in Indiana and my problem has started about the same time. I have an E150 with the 4.5L engine. In the mornings (7:00am) the van starts fine runs fine. At lunch (12:00) when I try to start it. It fires up then cuts off. At first when it did this. If I got her rolling it would do fine. Now it continues to cut off. It really has to be moving for a while before it will run on its own. Now the funny thing is, when I leave in the evening (sundown, 6:00PM) it runs fine again. This sounds like a vacuum issue to me that is being aggravated by the change in temperature. I would like to get other opinions and some tips on how to find the leak if that is the case.

Thanks in advance for any help provided.
Kevin
:-huh