Well honestly kinda odd, but a GT 630 is a decent mobile card so it should be fine for this. In League my recomendation would be to try setting the game to Ultra then clicking Stable and seeing what would happen (Or any graphic setting you were trying for). If its still spiking around, then you might have some display or performance problem. One thing to also check since this seems a bit random, your PSU for the laptop, is it getting extremely hot or can you watch your battery indicator (Or run on high performance setting on battery only) and see if it does it. If the Power Supply is overheating or has a short somewhere, your system could be randomly shutting it off from charging and dropping the performance to underclocked mode on the Nvidia GPU that is an auto setting in nvidia control panel. That underclocks the card and that could be causing your huge performance drops, check on that and see what happens.