I've had this car a few months and have been very happy with it. Lately though, it sometimes has a great deal of starting up. The starter motor seems to turn just fine, but the engine doesn't start. After keeping the ignition engaged for 10-30 seconds, the engine starts up and runs fine. This can happen right after I've been driving or after a day at the office. Doesn't seem to be temperature dependent. On the other hand, it usually starts up just fine.

I had the battery checked out at the mechanic and they said it was fine. I've also visually inspected the spark plugs and they don't seem to be fouled, but I have an inexpert eye. I tried disconnecting the spark plug wires while the engine ran one at a time to see if it made a difference in how the engine ran, and one of the sparkplugs didn't seem to affect the engine's running. At this point I'm not really sure where I should go in trying to troubleshoot the problem. Any help?