try removing battery & power, then hold power for 30 seconds on laptop, replace battery and power and reboot.
Also remove the Ram and replace it one at a time until sucessful boot. clear the CMOS by removing the CMOS battery and shorting out the terminals for 10 seconds (CMOS battery is typically a coin sized battery on the motherboard altho some are little packets and attach with a red and black wire on a small cable.
This actually sounds like a possible overheating situation & GPU seperating from the board. |To test this you would have to connect to external monitor and see if anything changes, if so then it's not the GPU, if no change and if nothing else succeeds look on youtube for the Oven Method of motherboard repair as a last resort.