Up and down the bracket in what sense?
- Stuck in Bronze IV with 50% win rate.
- Duo with a friend, win rate goes over 50%, climb to Silver.
- Solo again, win rate goes under 50%, fall to Bronze IV.
- Stuck in Bronze IV with 50% win rate.
If this is the way it happened, it perfectly supports what I was just saying: people who are at their correct MMR will have approximately a 50% win rate, while people at the wrong MMR will win or lose a lot until they approach their appropriate MMR.
I hesitate to mention it because I don't want it to seem like I'm picking on you, but there is even more evidence in your history. Over the past few days you duoed with someone who is currently unranked with high MMR. You played 8 games with him. In each game, every player except for you two is around Silver 2–3 or Gold V at most. Every single one of the games is a loss.
If you really belong in Silver, and if not having a duo partner is what makes climbing out of Bronze impossible, why did you perform so poorly in all these games? Are you really trying to convince yourself that you got unlucky 8 times in a row even when the odds should have been in your favor with a duo partner? Is that really the easier explanation to swallow, instead of the obvious one? You are not cut out to hang with Silver players. Maybe your cs is good enough, maybe it isn't. Maybe your mechanics are good enough, maybe they aren't. Maybe your game knowledge is good enough, maybe it isn't. But when you put it ALL together, your total skill level is too low.
And when you look around at other people at your MMR, they have the same total skill level but you can't see it. That guy whose communication is awful? You can't see that his pressure game is great because you've never heard of pressure. That guy who tilts hard and throws the team off? You can't see that his ability to zone in lane is so great that he always wins lane unless things go bad, and those are the only games where he doesn't know what to do so he tilts.
If you are interested in finding out what you are doing wrong and could do better, there's a whole section of the boards for that. There's 100 streams and 10000 YouTube channels for it. The whole goddamn internet is waiting to help you get better at League, and you're too preoccupied saying "Mmmmmmnnnehhhhhhhhhhh, he picked Yasuo support!" to accept that help.