Same problem here running Windows XP SP3. If I change to a different machine running Windows it works fine. A few days ago I reloaded the blink sketch from this other machine that would connect, let the board sit unpowered for a day or so, retried on the problem machine and it worked. Overnight, however, it has developed the same upload/connection problems.
Thanks for the suggestion. Can you explain what the "reset-diode-5V" connection accomplished?
please try the following fix and report back on how it works:place a diode between RESET and 5V on the POWER connector, with the cathode (end with the bar) towards the 5V. it will need to be a small-signal one, such as a 1N914, 1N4148. note that a 1N4002 or similar power diode will likely NOT work.