I have good communication between my 861 and my computer so I thought I would add a few thoughts. I have two serial ports and four comm ports available on my windows 98 computer. I personally use comms port four for my communication to the 861, as I dedicate that port to it. I remember a few years ago there was a software release that had suggested settings for the comms port. Of special interest was a suggestion to set the maximum bits per second to 38400 in the port settings section of the comms port used ( in my case four). I got noticeably more reliable results using 38400 and have used it since. For what it is worth, since I have two serial ports, I leave one hooked up and available for the 861 at all times.
The other thing that I do is always perform a fetch before doing a send. I find that if the 861 will successfully perform a fetch, it will perform a send immediately after it. If the fetch doesn't work, I turn the 861 off and then on and that usually does the trick.