I noticed the same thing. I was streaming great at 3500kb but then suddenly I was dropping frames when I started my stream in OBS. In OBS you can change to what Twitch server you stream to so I moved from the Miami server that I was on to the New York server and my bit-rate went right back up. Several hours with not a single frame dropped.
The next day I tried going back to Miami and still very bad bandwidth so I went to Texas this time and it was good. The very next day both Miami and Texas had poor performance so I went back to the New York server and it was good to go.
If your having this problem then just try to find a server that works best for you. Before blaming the Twitch server I did do a speed check at speedtest.net to make sure I had the bandwidth from my ISP first. I suggest you do the same.