2001 Mercury Marquis • V8 2WD Automatic • 42,000 miles

2001 Mercury G.M. 42K miles. Ran great but would not start/run after a brief stop @ the store. Starts but immediately dies out. Happened a few months ago and after a few hours it started right up and ran great till today. No loose connections seen. Plenty of fuel. Passed last emissions test. No dash trouble lights.
I have not run my OBDII yet, so no known trouble codes. Years ago this would have been a vapor lock but this engine is F.I. I'm at a loss for a cause. Any ideas?
July 30, 2011.

Thanks you for the info. I knew it had spark cause it would briefly start up but it then died out. As for fuel, it may have been a pump problem? I let it sit at the parking lot for about 90 minutes and returned with my OBDII to see what I could find. I tried to start it and it fired right up with out any problems. So I drove it home. No stalling or hesitation. This happened a few months ago and this was the exact same thing. I want to run it till it gets up to normal heat and then park it in the garage and shut it off and then try to restart it. If it will not restart, then I can get a reading of the trouble codes and see if it's a sensor that only fails when it's at normal temperature. Last time I checked the codes with the engine cold, it was all negative. Not a single problem so I'm thinking it needs the engine heat to recreate the cause. Could be a sticky IAC valve? I'll post my results ASAP.
Thanks again,

Jul 30, 2011.