UNO : sketch doesn't start at power on

I use UNO with MacBook(SnowLeopard) and Arduino 022.

When connect USB to UNO board (no external power supply), then orange LED ("L" silk) continues to flash, and the sketch program does not start.

I can write a sketch in this state, and the sketch works after writing. However, when unplug and re-connect the USB, then LED continues to blink again, and the sketch doesn't start.

(I think, AVR's EXTRF bit in MCUSR does not clear, somehow.)

This situation occurs at external power supply, too.

What should I do?

if you press the reset button, does the sketch then run?

if so, try placing a diode between RESET and 5V on the POWER connector, with the cathode (end with the bar) towards the 5V. use a 1N914, 1N4148, or similar small-signal diode.

No, the sketch doesn't run by pressing the reset button. I tried placing a diode , but the sketch doesn't run too...