CO-OP is unplayable for me and friend

Instead of easing my way into the explanation of this problem I am just going to flat out divulge the details. Tonight was the last chance that I was willing to give the Master Chief Collection because of the sheer amount of times that I had been disconnected from matches and legendary missions. A friend and I had been going through every campaign on legendary co-op and though we had connectivity issues that made some levels unbearable we stuck with it. We managed to complete Halo 1 and 3 on legendary. It was when we went to complete Halo 2 that the insanity started. On three separate occasions near the end of a legendary level the connection dropped and forced us out of the match only to have us pulling out our hair and having wasted most of our night. Tonight however, after having taken over a month break because of that we decided to start up again. But the mission we loaded up wouldn’t even allow us to play together. Either me or him would not be moving on each other’s screen or I would move way ahead in checkpoints and he would be nowhere to be seen. I ended up giving up and am now writing on here. Something needs to be done! I have wanted to play Halo and enjoy it but it is being made impossible. I am an achievement hunter also and this is maddening. What can be done about this?