Uno SMD not running sketches unless reset

I have looked around a bit for information about this and find that the issues discussed are only for the Diecimila and earlier. I have the Uno SMD. I have uploaded a sketch and connect external power, but the sketch doesn't run unless I hit the reset button. The same sketch once worked under the same conditions, but now requires the reset to run properly. I have also run the sketch on a Uno R3 and it runs without needing to be reset. So I know it isn't the sketch, but the hardware. How can I get my Uno SMD to run without having to hit the reset button every time I power it up?

mitaccio: I have looked around a bit for information about this and find that the issues discussed are only for the Diecimila and earlier. I have the Uno SMD.

Really? This was a well known issue with the Uno SMD: http://arduino.cc/blog/2011/01/20/issues-with-the-new-arduino-uno-smd-edition/

You need to update the bootloader on your SMD.