Staff Casting Lag?

I’ve been having a problem, ever since I started using a staff. Whenever I start to full-cast an attack using whatever staff, there a chance that it gets interrupted. I dunno why it’s happening as my game’s running well. It mostly happens when there are over 20 enemies on screen or there are many things being shown on the screen or i’m walking on a stair or anything slanted. Is it lag? Or is it something else? Is anybody here having this problem, too? I get killed when enemies overwhelm me, because my casting gets interrupted quite often.
Now i’m just half-casting just to be able to CC the enemies.