Can't say for sure but that sounds like it could be the coil, I had a similar issue on a few occasions a while back.
First I thought it must be the main relay and then it didn't happen again for a long time so I thought it must've just been some bad gas but there were a couple times after that when it didn't start up "normal".

Finally my coil died - started up fine and idled for 5 seconds before it quit - and since replacing it I haven't had the issue again.

P.S. Your tach isn't reading noticeably off is it? If it was, which mine had been doing, that would all but confirm (to me) that it's something related to the coil or ignitor/ignition module.