When it works When I'm running the blinking led sketch and powered by USB, the LED connected to 13 works as expected as flashes every 1 second.
When it doesn't work When I power the board with an external 9v 1000mA wall-wart, and run the blinking led sketch, the LED connected to 13 flashes very rapidly and definitely not 1 second (like 5x faster).
Problem discovery I realized the problem after I was unable to get my WiShield to connect when powered by external power.
My Setup The arduino is brand new. I'm running Arduino 17 on Mac OSX. Last week, the IDE crash and was unable to upload sketchs with a programmer error (i think). The problem appeared to work itself out after a restart and reset of the arduino.
Plead for help Has anyone else ran in to this? I'm not really sure what to search for and have come up empty handed with searches like, "rapid LED flickering" and such. Thanks for any help!