Tried that, didn't work
Well then I'm stuck for a cause. You said the chip works in the first board and the loaded sketch runs if placed into the second board (proving the chip has a good bootloader) and if the sketch uses serial communication it works in both boards (proving the USB serial converter is functioning).
So I'm out of possible causes for those symptoms, sorry.
EDIT: Maybe one last test, run the loop back test on the suspect board (it's a sticky around here somewhere) cause perhaps the USB serial is only functioning in one direction, and your 'test sketch' only sends data to the PC but doesn't test the ability to receive data from the PC?