my advice: use 9600 baud.

9600 baud was the only reliable rate for our P89 at ~12Mhz. it does consume some time writing to the flash memory. while you may have been able to run some serial data IO at higher speeds, its not limited by cycle times.

maybe your utility or cabling doesn't support xon/xoff protocol, rts/cts whatever. probably not worth resolving.
duh