Why does my laptop monitor flicker when i'm using a netbook charger?

I bought a general netbook charger which has a 19 V DC output and it is compatible with my 19 V laptop. The outlets are likewise compatible. It charges okay only after a few mins it seems like the power fluctuates and the screen flickers. The netbook charger says it has up to 40 W muscle supply and the laptop chargers up to 90 W.