Problem solved  - Diagnostic all came back good.  I found the solution.  I went into the BIOS and under video and disabled "Dynamic Backlight Control"  and now works perfectly.  Only took 5 months to find the fix.
HI I was wondering if anyone has solved this issue.  I have Dell XPS 13 9370, I replaced the standard fHD screen as it was cracked with a touch screen.   When it works it works well, however I have several issues. 1.   On cold start laptop screen will not work until connected to an external monitor then it is fine. Whenn synced all works well. 2.   Returning from sleep it sometimes does not work until connected to an external monitor. 3.  Touch screen works well   I have done following: a.  Using latest version of Windows 11  b.  I have disconnected the battery and then drain any remaining charge.  Works for a while then back to normal. c.  I have updated the graphics driver from intel latest version UHD 31.0.101.2111 - it is the same (removed all old drivers first).  Clean install   Any thoughts as to a possible solution?  could it be a battery issue?            
Dell XPS 13 9370 replacement display issue