First, about what works: Arduino (I think it was rev. 1 or 2) and xBee Pro 868MHz: I connect Rx and Tx pins of Arduino and xbee (Rx->Tx, Tx->Rx) and power xbee with external power source (3.3V, the same gives 5V to Arduino). When I connect external power source it powers both xbee and arduino. Serial communication works via xbee. When I connect USB to Arduino, it powers only Arduino, and serial works beautifully via USB. Everything is OK.
Now I need second device, I bought one more xbee and Arduino (it looks slightly different, but it's also rev 1 or 2, definitely not 3). Connected everything the same way. But now when I connect it to USB, serial connection doesn't work. Well it works sort of, but its very unstable. At the moment I don't know how to debug it and get some more info what works and what doesn't.
Any ideas what is wrong? Maybe I should add some optocouples on Rx/Tx pins?