I read some few things while using an IO pin as input and using serial communication that creates some trouble . . ???
If you are talking about pin0 (rec comm) on the arduino, it is already pulled-up via the 1k resistor wired to the USB serial converter chip, so it will not see a 'floating' input condition.
Furthermore, my Arduino was powered through my usb, i just transfered to an external power supply (9 Vdc) and get my code running as perfect, and forever . . .
Any explanation on that ?
It could be (most likely) that your full setup of external hardware is just drawing too much +5 volt current. The USB 5 volt power coming from your PC is limited by the on-board thermofuse, rated at 500ma max. If however you are running using external power then the on-board 5 volt regulator is supplying the current and it has a little higher max current rating then when using USB power.
Lefty