Is my SparkFun RS232 Shifter SMD faulty?
The sparkfun is a bit of a hack device that doesn't actually do proper RS-232 level shifting.  Instead it uses the voltages from the other end to charge capacitors to use for the sending voltages - a clever idea, but I can forsee flaws if the other end isn't sending high enough voltages...

I would recommend ditching the sparkfun and using a MAX232 or similar device to do real level shifting.