XPS 17 L702X Power Issue?

Hello, my apologies if this has already been queried, but I couldn't find anything on it!

About two weeks ago I purchased a second-hand XPS 17 L702X laptop. I was thrilled with it. It's a beauty and runs very smoothly. It did have a third party charger with it, but the laptop was demonstrated before my eyes and everything seemed to be running smoothly.

And it does! Mostly. The charger itself works; it fits in firmly and it charges the battery.

This is when the fun begins! When I am in a game or in something otherwise intensive, the charging light flickers white. It's only when in that kind of situation; when just browsing or using an office program everything seems to be fine. 

Could it be the charger? 

I've had a little look online but there's nothing I can find, I bow to your wisdom Dell Community!