I've had this problem twice in the last two weeks and am not sure what could be causing it.

Both times it has happened after I have been driving my car, I'll have it parked and shut off for roughly 15 minutes and when I go back out to start it it will start, shudder and then die. The first time it happened it did it three times, the third time I threw it into gear before it died and it was really shaky until I got a little speed and then it ran just fine. The second time took several attempts, 10 or more, and even throwing it into gear did not help anything this time, once it finally started it was very shaky like the last time, I noticed that when it was running before it would die it was only at 500 RPM. For about 15-30 seconds after I got it running and was driving out of the parking lot of the store I noticed it wouldn't go below 1500 RPM. It was a little shaky again at the stop sign but ran just fine until I got home, put it in park, shut it off and then tried to start it again. The engine turned four or five times before it started back up, then I shut it back off. I just went outside and turned it on, the temperature gauge was about a little over a quarter of the way up (usually is right under half after it has been running for a while) and it started up no problem.