David, what Dan is trying to do, is to determine if it is your video card/circuit or your LCD. So the best and quickest way is by attaching an external monitor. Here is what Apple says about your symptoms:"
Quick Check
Symptoms Quick Check
Vertical/Horizontal Lines
• Vertical lines
• Horizontal lines
1. Boot from install DVD to determine if potential software issue exists.
'''2. Verify if issue is visible on an external display.'''
Deep Dive
1. Boot from Install DVD and verify if issue is still visible:
• Flickering image
• Horizontal bands of static intermittently appear
• Dock and / or menu bar shift positions
• Display intermittently flashes on / off
Yes--> Go to Flickering issue, go to Noise / Unstable Flickering.
No--> Issue likely caused by software or driver issue. Troubleshoot for software
issues. Make sure all software updates have been installed.
'''2. Verify if issue is visible on an external display attached to mini DisplayPort.'''
Yes--> Issue NOT caused by internal LCD. For systems with a video card, go to step 3. For systems without a video card, go to step 4.
No--> Go to step 6.
3. If a video card is present, remove video card and inspect connector for damage or corrosion then reinstall. Verify if issue still occurs.
Yes--> Replace video card.
No--> Issue likely caused by poor video card connection.
4. For systems without video card,horizontal lines may be related to a failing RAM module. Verify if video issue only happens AFTER Apple logo and spinning wheel appear.
Yes--> Issue only happens AFTER Apple logo and spinning wheel appear. Go to step 5.
No--> Issue present before Apple logo and spinning wheel appear. Go to step 8.
5. Start with shift key down (safe mode) to disable system extensions. Verify if issue still
happens when booting in safe mode.
Yes--> Go to step 8.
No--> No video issue when booting in safe mode. Go to step 6.
6. Isolate with only one memory module installed, then with the other one. Test with knowngood
memory. Verify if issue only happens with specific RAM module(s).
Yes--> Replace affected RAM module.
No--> Go to step 7.
7. Isolate with one known-good memory module installed in one memory slot. Repeat by testing in other memory slots Verify if issue only happens with specific memory slot of logic board.
Yes--> Replace logic board and retest.
No--> Go to step 8.
8. Disconnect DisplayPort cable between LCD display and logic board, and verify if there is damage to cable or connectors.Repeat with vertical sync cable connected between LCD panel and LED backlight board.
Yes--> Replace DisplayPort cable and vertical sync cable or any other damaged part/module.
Go to step 9.
No--> Reinstall DisplayPort cable and vertical sync cable.
Go to step 9.
9. Verify if issue is still present after reinstalling internal DisplayPort cable and vertical sync cable.
Yes--> Replace logic board. If video card was present, reinstall original video card.
Go to step 10.
No--> Issue resolved.
10. Verify if issue is still present with replacement logic board installed.
Yes--> Go to step 11.
No--> Issue resolved.
11. Reinstall original logic board and verify if issue is still present with original logic board reinstalled.
Yes--> Replace LCD panel. Run Apple Service Display Utility.to verify the resolution of intermittent display flickering symptoms.
No--> Issue resolved. If a video card was present, reinstall original video card."
So, I strongly advice that you follow Dan's way of doing things. Good luck.