This is a known problem among Thinkpad T400s users, and has been commonly linked to a faulty connection somewhere along the LCD cable (connections/cable itself):
- At the terminal in the display itself
- Along the cable which is a result of it being “pinched” or crimped
- At the terminal on the motherboard
If you notice the flickering resolving itself after flexing the screen (lightly) or that the problem only occurs in certain orientations then you are likely experiencing a wiring issue. If the screen only flickers at the start screen then it is possible that your drivers are out of date and you will want to update those before continuing.
If you are convinced it is a hardware issue then you can remove the LCD bezel (the plastic rim that surrounds the screen) and check the terminal connection at the display. If that doesn’t resolve the issue you will likely need to go further into the guts of your laptop and check the connections at the motherboard terminal and along the cable. If you notice that the cable has an issue then you will likely need to replace it. If that doesn’t resolve the issue then your LCD screen may be faulty and needs to be replaced.