Hi Bebop
I would not recommend dismantling a Laptop as they are a pain to work on, however if you’re feeling brave the support section of the manufactures website usually has details of how to dismantle. (Just make sure you follow the instructions religiously)

From the descriptions you have given, my guess would be a dodgy connection, in particular in the ribbon cable that connects the Motherboard to the screen, (The fact that the laptop will work with an external monitor when the internal one disabled, proves the internal system is the problem) which looks like it may be shorting out.
A reputable repair engineer should be able to identify and fix it at minimal cost.
Hope this helps

Bill