Not had these issue with my line up, however had issues with flashing software. (K3 Bricked)
As Engineer having updated software "all my life" and having experience over here with the K3 - P3
I suggest to disconnect all cables *except for power), put serial cable in K3 only...flash required firmware.
once done take cable put in P3 only ( make certain it is powered though) flash required firmware.
Make certain K3 seems to operating normally.... if Ok then hook up P3. nothing else, check for improvement.
In case all ok, add additional monitors and whatever you fancy.
In case nothing hels, there is always Elecraft Tech support., they usually give a rapid response.
Just my 2 cents.