Question M4600 turns off under load?

Feb 6, 2020
I recently just picked up a nice m4600 from this owner off eBay. He's been using it for minor rendering tasks and it worked great.

I bought a new battery (OEM) and a caddy for it and it was off to be used as my new laptop (I like older workstations). So I stress tested it and when plugged in for 12 hours constantly rendering a pretty big Cinema 4D file, it was great! No issues, full performance, no drops in boost, and temp was pretty cool at 80C (Load, fans spinning, yada yada).

When I go into my bedroom to watch some Netflix I was watching for about 10 minutes then it shut off. It wouldn't turn back on until I removed the battery and put it back it. It has done this 7 times now. It's fine when web browsing (but it has shut off once) and it doesn't overheat. Did a diagnostic and it passed all the tests in Dell's diagnostic thing.

Plugging it back in fixes all the issues, and I was able to game on it with some Rocket League, TF2, and Doom. I was able to render a file in Cinema 4D and Adobe Premier CC 2019. It was able to watch a full 4k movie for about 4 hours and no sweat when plugged in. When it's not however it lasts for about an hour with moderate browsing, and 10 minutes when anything with a load happens (Cinema 4D, YouTube/Netflix video, Adobe, Gaming, even Wallpaper Engine sometimes.)

Is the battery to blame? Motherboard? GPU? IDK. It never shutoff when plugged into the wall. Thanks for any help I receive!