You've def got some issue with video config between your internal laptop screen and the external.
Turn on EDID and then review the log closely after booting - pref with another machine SSH'd in remotely.
In you /etc/X11/xorg.conf file, you will find:
Option "UseEDID" "false" twice.
Change to true. Now read /var/log/Xorg.0.log
Read the mode validation stuff and note it distinguishes between internal and external DFPs. Read through all the stuff it discovers when probing these looking for warnings and errors, but also which output and mode it ends up selecting...