Bitrate bottomming out is usually due to a network issue rather than an encoder issue (otherwise, OBS will report errors). Ask your ISP if your connection is stable, if not, ask them to fix it. Some (really) crappy modems may also have downtimes to clear cache and reset gate arrays occasionally, which may also cause what you're describing.
Even if you have a stable connection to the internet, congestion to your stream service from your ISP or intentional throttling could cause your stream connection to be unstable. Routers may also improperly handle QoS packets. I wouldn't however recommend disabling QoS as Apple devices require this to function. Set your stream to the top QoS (Type the address of the RTMP server you're streaming to in the first priority for QoS). If you're streaming through wifi, set beacon interval very low, set first priority to your streaming computer. Give your streaming computer a static address or set DHCP lease time to refresh daily at a time when you won't be streaming.
If it happens very briefly (a few seconds downtime), a quick work around could be to have your streaming service (YouTube, Twitch, NicoNico, etc...) buffer your stream, and set OBS not to drop frames. This will give viewers a smooth playback despite your connection issues. This can be done on YouTube through selecting the option - Optimize for less buffering. This can be done in Twitch by setting a delay in broadcast settings.
To calculate how large of a buffer you will need (minimum): Buffer = (( Network speed - Bitrate ) * Downtime / Bitrate) + downtime
For example, if I have 10mbps upload speed and I'm streaming at 4mbps with occasional 5 second down times, then the buffer I will need is ((10 - 4) * 5 / 10) + 5, which is 8 seconds. Add a few second of extra buffer to the minimum required.
The first stop is still to simply CALL YOUR ISP. The solutions/workarounds listed here are for when your ISP refuses to help or is entirely incompetent. If you don't understand how to do any of this from the explanations provided in the second paragraph, then it's probably best that don't mess with these settings.
