I started a thread on this topic about 1-2 years ago which ran to more than 100 posts. There wasn't an internal solution, ie changes to Windows or OpenCpn settings that worked.
But XPort, a freeware com port redirection program easily solved
the problem. Google
it up and dowload it. Redirect your current
port to something unused like Com 1. Then set OpenCpn to com 1 and you will have a solid connection.
You have to rerun XPort after each reboot, but it is a small inconventience that really works.