I was having a similar problem using the HW kitchen board and the telit GE863-GPS module. The telit was configured for SELINT=2, #SMSMODE=0 and +CMGF=1 (Text mode). I was able to send SMS that were 160 char long without any problems. However, one corrupted message contained some characters with the 8th bit set - 8 bit chars. When this happened, I received the ERROR 305 - invalid text mode parameter. Once I masked out the 8th bit, the error went away. You may want to double check to make sure that no 8 bit chars are in your message stream - IMHO.