Rapid (almost rythmic) flickering until eventual vertical lines.

Found a pretty nice 55" TV the other day in the trash, made sure it was dry, hooked it up and watched some Netflix on it, sounds and looks great so far. So I figured "Sweet I got a free TV" but after maybe 20-45 min (depending on the mood of the TV) the image starts to flicker but not on off, instead it kinda bounces the image a bit then stops. Then starts again and repeats this process until it eventually just becomes nothing but vertical lines. So I did some research and figured "okay maybe it's the T-Con, it's pretty cheap anyways so I might as well replace it" so far it changed nothing. I'm starting to suspect something might be the matter with the main board because when I hooked up my apple TV to the HDMI ARC port it started to make creepy tazer-like sounds. So something must be shorted to ground I assume? I just don't know what. And would the main board cause vertical lines like that? I'm stumped. Anyone have this issue before?