More than likely your stop bit rate is wrong. will also happen on xon x off
goto your machine pull up all the params for the stop bit and baud even x on x off and match them with your pc program.
Did this just start happening? if it did first do a cold reboot on both your pc and the machine, if it still persists than something was changed somewere along the lines pc more than likely.
if you never tried it before you need to set the parameters for both the pc and the machine the same.