Help with a Problem, 2018 Precision 5520 4k?

I have a problem Playing YT vids on full screen, but only when the laptop charger is plugged in. The Vid playback becomes slightly laggy. On the small YT playback screen, there's also no problems. Unplug the laptop charger, YT vids in Full and in 4k playback perfect. Plug the charger in, straight away, YT vids become laggy. Take it back out. Vid play perfect again. I thought it could be the or charger. or battery? I ended up buying both around 1 week ago. Battery is a 54w. 

Here the specs 

Running Win11

4k touch Screen on board 4gb Graphics card

Intel(R) Core(TM) i7-6820HQ CPU @ 2.70GHz   2.70 GHz 

1TB  NVME SSD Read Speed 2240mbs. Write speed, 1700mbs

32gb Ram

Any advice appreciated

Cheers All