There is a TSB for a NON-START condition after a "heat soak".
Basically, you have been running the van, and stop the engine for about 15 minutes, you come back to start the engine...and it will not start.
The cause is moisture in the fuel pump relay...which is inside the CCRM (Constant Control Relay Module) on the '96. Later, I don't know what year, they moved that relay into the power distribution box in the engine compartment.
The solution was to replace the CCRM with an improved one, with better sealed relays. The CCRM is located in between the battery and the radiator.
I have pictures of this in my pictures at
http://community.webshots.com/album/201931518cScpNK
I don't know if this can also cause the rough running engine under the same conditions....but possible. I have had the same issue as you....and it has not done it at all this winter....but what fixed it?.... I don't know.
Another possiblility is a coolant leak, on the 96 the head gasket is not as high of a failure item as the '95. The lower intake manifold gaskets can leak....permiting coolant into the intake.
If you are loosing coolant (monitor the overflow bottle), then I would check into this (I put Bar's Stop Leak into the radiator to solve my several slight leaks, timing cover and lower intake manifold gaskets).
If you are not loosing coolant, then don't worry about this possibility.
You might also verify that the coolant level in the radiator (make sure there is coolant in the upper radiator hose...which you can tell by squeezing it gently). Just to be sure that the overflow bottle is still in the loop....
I noted that there was no clamp on the overflow bottle hose...where it connects to the radiator......and, while it was not an issue....I found that it was kind of loose. Also, only takes a little bit of something on the cap seal or where the cap seal seats.....to cause the radiator cap to not hold pressure.......simple wipe off solves that.