Sounds like a connection problem to me, my brother plays on the ps3 and only ever has a problem if our internet is acting up. I would just say keep your wits about you and pair the party list and focus target close together.
T5 requires a lot of attention. my biggest problem was getting caught watching the party list without glancing at my toon. Very easy to get tunnel vision.
Oh and buy a headset doesn\\'t have to be an uber fancy one those noise cues help a lot