1) the software UART will not work if other interrupts happen, the time to process those will upset the timing of the soft UART, by a fixed amount. This could be the reason that your soft UART works at 2400, but not at 9600. Make sure that the interrupts used by the soft UART have the high priority and all others have low priority. If you have found a soft UART that does not 'clock' by interrupts and you are using interrupts at all, all bets are off.
2) make sure that the timing constants in the soft UART you are using match your clock speed.
3) a known working software UART can be found at http://www.semiconductors.philips.com/acrobat/a.../AN423.pdf

Erik