I'm using a Rev 1.4 display, controlled by our custom embedded control board) and sending 2 bytes at a time (we have been using our control board to send text to Hyperterminal for some time with no issues).

I did have the LCD displaying text (2 characters at a time) no problem, and was able to ...

Could you remove all jimpers & create a circuit that switched between TTL & 233?

I'm having a similar problem - eg sending the "Backlight Off" (0xFE 'F') command results in "F" being displayed.