In regards to your comment on openhab/openhab-core#1842:
I am wondering if this is actually a permissions issue
Is your openHAB user a member of the dialout group? I.e., if you look in /etc/group, does the line for dialout list the username of your openHAB user? For example, on my system, my user is the only member of the group:
$ grep '^dialout' /etc/group
dialout:x:20:mrdos
But permissions are not typically responsible for intermittent behaviour.
If I use a python script to read from the port, that works just fine.
Is your script running as you, or as the openHAB user? If you run the script as the openHAB user (i.e., sudo -u openhab python serialtest.py), does it still work?