Why are you creating a trigger for that unit, anyway? You could just change the Acquisition Range of the unit in particular to the range you want it to start chasing units. That way, it would save you all the bother of triggering it when you don't need to. Also, it will only chase units it can see, so you might want to increase its Sight Range too, or just make it the owner has vision of the entire map or something. Good luck, and I hope this has been of help to you.