i am not sure how i would try software serial on a 328? the board has the FTDI chip hard wired to TX and RX, at the moment i am using the board with the chip removed just as a serial converter to drive the ATtiny. if i use the original 328, won't the hard wired tx and rx pins automatically interrupt the processor even if i jump those pins over to a software serial port?
I did not realize you was using the arduino board. That seems to rule out hardware problem unless the patch cables to the ATtiny are long/noisy.
I'm not sure about the core of arduino but I would expect the hardware UART pins only gets configured and used if you do a Serial.begin otherwise the pins should behave just like any other pins.