I'm convinced I have the same problem. A member of my raid team is a good PvP player- I've never had a good match playing on his fireteam. I can play with others and be at the top of the match- but never with him. I have another friend who says she just can't play with him. I think in her case she thinks it's because of SBMM, but in my opinion it's something to do with his connection.