Weird laptop display problems

I have a Sony Vaio laptop from a friend. It has a weird display problem. The colours are all wrong - pinks, blues greens, text is indistinct, swirly coloured lines prevent the screen content being read easily.
If I connect an external monitor, the display on that is fine. This suggests that the mobo/GPU is fine.
An odd twist is that, if I start up a CMD screen, THAT part of the display is fine on the laptop screen - a rectangle window of clarity on a desktop of swirly, miscoloured screen. Odd.
If I THEN plug in the external monitor the display is still ok on the external but the CMD screen THEN takes on the same faulty display characteristics of the rest of the screen. The CMD windows returns to normal if I disconnect the external mon.
Any pointers? I'm guessing inverter or bad connector but, before I strip it down, any thoughts?
Thanks

The problem is still evident when in BIOS and also after booting into safe mode. Careful inspection shows that the CMD windows (although I thought it immune from the problem) is also affected but much less evident. But the problem in the CMD windows DOES get worse when you plug in an external monitor....
I guess the above rules out a graphics driver problem.
Likely a HW problem. Any thoughts?
A better description of the problem would be: blurred text, unusual colours, flickering pixels, random green pixel dots across the screen, "smearing" is the right word for the general look of the screen...
Thanks for any advice...

Thanks. Will report back following stripdown!