Inverted is on screen problem

Im having a bit of trouble with npcs in my project. Im trying to set up so that if an npc is off the screen for 3 seconds, they get destroyed, the problem Im having is that I dont know how to set up a check so the counter resets and stops if the npc is on screen, at the moment they blink out of existence while on the screen. Could someone hint me on how to set this up properly?

Add a Timer to the NPC.

Every 0.5 second
NPC is NOT on screen
NPC is NOT timer "destroy" running
....NPC start timer "destroy" for 3 seconds

Every 0.5 second
NPC is on screen
....NPC stop timer "destroy"

NPC on timer "destroy" 
....NPC destroy