I have had trouble with the VISA drivers when using a serial port. The problem I encountered was the string sent by each function did not have a terminator (\n) built into it. Appearently this terminator is optional on GPIB, but needs to be there on Serial. I have modified some of the VI's and added the terminator character and they then worked fine. I did not re-try them with GPIB to see if it affected them however.
I hope this helps.