I've done a bit more testing today. The old QB64 works fine at 115200 (needed to set the higher speed in the windows control panel) and is, obviously at that speed, very fast and accurate. With the newer versions I end up with lost and incorrect data and it runs much slower.
Perhaps I should have put this in the "bugs" sub-forum because that's what it is seeming to look like.
Does anybody else use serial COM ports? (Of course this is actually USB pretending to be a COM port, but still should work)