Umm.. you said you removed the keyboard, and the ribbon. Am I to assume you have dismantled the unit before?
Does your laptop work 100% fine, but start to artficact when it warms up?
Connected an external screen, to see if the problem persists?
Some Steps for now:
Take the ribbon off again, and lightly scrub the terminals with isopropyl 100% with a dental brush. Then reattempt.
Also, try tapping the back of your screen instead of the palm rest, see if the artifacts change even MORE. Try to zero in on the optimum location that affects the screen.
Check GPU temperatures as well.