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How to stop FFMpeg slowly dropping Bitrate in RTMP stream (To YouTube)

I’m attempting to stream to YouTube from FFMpeg on a ubuntu server – I’m trialling it on a 2 VCore system with 4GB of RAM – I’m attempting to stream to youtube, with a bitrate of 4Mb/s, at 25Fps, at 1280×720 – the stream starts off at a normal bitrate at standard speed, but while the bitrate and fps continues as normal, the bitrate drops continually over the following 10 minutes, dropping down to 800Kb/s at a low – what could be causing this? The command used is below:

ffmpeg -re -i "TestStream1.mp4" -c:v libx264 -b:v 4M -s 1280x720 -c:a copy -strict -2 -flags +global_header -bsf:a aac_adtstoasc -bufsize 4000k -f flv rtmp://a.rtmp.youtube.com/live2/<stream-key>