Four things to check when having problems with serial com:
1. Make sure the baud rate matches the device.
2. Make sure the parity bit matches the device.
3. Make sure the number of stop bits matches the device.
4. Look for buffer overruns if you do not use flow control.
Like CC said, using Hyperterminal will verify that you are getting good responses from your device. If it runs fine for a while, then you start getting errors, the most likely cause is buffer overruns. You will have to use flow control to prevent this from happening. You will have to know what type of flow control your device supports, hardware flow control (RTS-CTS), or software flow control (XON-XOFF).
CC: Look again, he did post his vi.