2002 Toyota Sienna Engine dies after starting.

I've noticed for several months, when I start my 2002 sienna van in warmer weather, it starts just fine, but when I engage engine into Drive, it dies. It sometimes will take me 10 minutes to get the van started. During the winter, I don't think it ever happened, but it happened late summer 2012, and now in warm/humid spring 2013, it's happening again.I've tried revving-up the engine, letting it idle/warm-up, and none of these seem to help to get engine started. Generally I just keeping restarting over & over, & after 10 minutes or so, the van will run.3 or 4 times, i've been able to actually move, but when I drive a few feet, the van dies. It died twice after driving 2 or 3 minutes, which was real scary, as I had to pull off the side of the road.I've been stuck in Kroger parking lot, and my driveway, several times, trying to get the van started. Any suggestions!