Windows 7: gray screen with horizontal lines at start up or wake from sleep

My daughter's hp pavilion dv6-6163cl entertainment notebook has begun to show a gray screen with horizontal lines at start up and after sleep (opening the laptop cover). This lasts for a few moments, then her log in screen appears. There is no sound when the screen appears and no mouse visible. It does not happen every time the computer starts or wakes from sleep.

No new software installed; nothing downloaded. Microsoft Windows is updated.

What I have done already:

updated to latest Intel graphics driver
hp diagnostics test on hard drive and memory (via BIOS screen): both passed
Windows Defender Offline scan
Microsoft Security Essentials virus scan
sfc /scannow
chkdsk /r
safe mode and restart
"windows repair" after pressing f8
selective start up
malwarebytes scan
Spybot Search & Destroy scan
uninstalled McAfee and installed Microsoft Security Essentials
System restore to earlier time
BIOS driver updated

Haven't been able to tell if the problem happens in safe mode because it happens so intermittently.

Any thoughts are appreciated.

I think it is a connection problem between the laptop monitor and the base--my daughter just showed me the display flickers when she moves the monitor. I've read about faulty or loose wiring between the monitor and the keyboard/base of the laptop.

I'll try to see if we can get this fixed.