Either you're on an outlier of hitting bad teams at the wrong times, you remember the good ones in non promos but not the good ones in promos, or something is changing in promos for you. I guess we could go w/ your theory that they are rigged and the moon landing was faked, but there are two other completely probable explanations I think you're overlooking in how upset you are about losing promos.
The first and more happy concept is pretty simple. You're stressed about promos and /or the experience you have had recently with losing them. This leads you to play different. Perhaps you play worse because you're stressed. Perhaps you play just as well, but are more irritable and talk down to your team at the first sign of trouble and they themselves play less serious (I know when I finish promos to a new level I am less inclined to work hard to carry someone in a game they are being rude, I lose no LP if we lose so I don't care much. Since you're with people in X5 of the tier above you this may be the issue if you're on edge and get mad at them easily.). It could be different things, but if the stress of promos and your recent luck changes your behavior then it may be what is making the difference.
The other and less attractive proposal is that you are about at the MMR your skill level is represented by. If you're in the MMR range where your true skill level is then you should be able to win games to get to 100LP reasonably easy, but when placed with/against players of higher skill level when trying to jump to the next rank it is more difficult. Your play changes as a result of the skill disparity and it impacts your team. This may be less likely if you're winning lane and team is losing lanes hard every game (make sure to account for CS, had someone raging from top lane the other game because his lane was the only one "not losing" and he was up by 1 kill top lane, and down by over 60 CS. His lane opponent was one of if not the most fed on the enemy team and did the most damage in team fights.), but if that only happens some games and you're losing mid or late when you won lane and team went even or only slightly behind then that's probably an issue w/ your macro play and your team fighting and objective play isn't strong enough to be carried by the small level of how far ahead you are in lane.
Not sure if that's useful, but I really do hope that it's just you're stressed in promos and when you can look at what you're doing different in promos you can easily change it and make it to the next rank. Good luck and I hope it turns around for you.
