Off screening questions regarding minion AI

I notice that my frost sentinels and very fast SRS will sometimes clear many screens away, other times they do not attack those offscreen monsters until my character gets close. Does anyone know why this happens, or have a way to remedy this?

Or a related version of the problem: You spam minions (eg hard hitting skeletons) in a location with a lot of monsters and they get to killing, but if you walk away they just stop attacking. You map for a bit, then return, and the skeleton is still sitting right next to a low level monster. As you get close, he comes to life and whacks it. But .... not all the time. Other times you plant skellies, run on and by the time you return they have killed everything in that room and run on to the next room while you are many screens away.

Next question: Why do ranged spectres sometimes stay really close to you, other times stand and fight until you leave the screen? There is a monster right next to my frost sentinels. Sometimes they will stand there and cast spells while I am running around in circles, or running off the screen. Other times, they will ignore the monster(s) and spend their time running in circles with me, or follow me like glue as I run off screen. When they follow me, they deal no damage at all, so become useless.