won't start

What is your question?

There are a few times where the Uplander won't start. I actually have to rev the engine to get it to start. Any idea what may be causing this? It's under warranty but when I take it into the shop it doesn't do it so they can't verify that it does in fact not start at times so they can't fix it. If anyone has any clue as to what may be causing this and how to fix it I would greatly appreciate it. It also does it in all kinds of weather so it's not just in the cold or anything.