With a third party port I don't know how much we will be able to help if the close is getting jammed up. What you are seeing in Portmon is the call to the driver itself--at a lower level than NI-VISA. The only recommendation I would have is to make sure that you don't have any pending operations, such as registered events. Make sure you unregister everything before you close the port.
It would also be good to try this on another type of serial port to see if the problem only occurs with the particular adapter you are using. Do you have a built-in serial port you could try it with?
-Jason
