Windows 7 64 bit
Generally speaking, I think it's really important for posters to tell us exactly what system they are using. Sometimes it makes a difference to people who would like to help.
One final question, and then I am done for this thread: Does the symptom persist if you unplug the UNO's USB connection and power it externally?
Anyhow...
I'm sorry to have wasted the board's bandwidth on something that couldn't help you. But: See Footnote.
Regards,
Dave
Footnote:
Putting in the delay() that keeps away from micros() is, at best, a Band-Aid that masks the real problem: A system bug that causes the Bad Thing to happen.
What I forgot to mention is that the UNO was absolutely unusable on my Linux system until I updated the firmware in the USB interface chip. That made it "almost good enough" for casual testing, but it still has problems that I simply can't reproduce with the FTDI interface on Duemilanove boards or other designs that use the FT232 chip for the USB interface.
For Linux users, I still think it's a matter of interaction between the Bootloader and the Operating System's treatment of /dev/ttyACM0. Whether it can be fixed with a new load of the bootloader or updating firmware in the ATmega8U2 USB chip on the UNO or a change in the Operating System's handling of USB with /dev/ttyACM0 is still awaiting discovery and fix.