Mob targeting without sight.

I am making a mob that will always chase you down. I've already put together the code that despawns the mob when it's too far and spawns a new one in a radius around the player but I need it to always know where the player is. For now the mob needs to notice the player to begin chasing down after each respawn. Aditionaly his path finding is very wonky. Most likely because he is fast. He turns back, walks the other way then turns back to the player again before he resumes chasing for no reason even on straight terrain. So is there a way to fix that? Additionaly I coded him to leap up if he can't reach a player. He does jump but never get's on to the platform because he desn't have forward momentum mid jump.