Doesn't so much matter if you're streaming at 480p, 720p, 900p or 1080p, as much as what bitrate you're sending, to which server, and if you're casting within your actual bandwidth (aka: forget what speedtest.net tells you, it's worthless for livestreaming).
Do post a log. It's possible that you're lagging or dropping frames instead, which looks laggy/stuttery. They're in %appdata%\OBS\logs and are timestamped. They're textfiles, so just open one with a notepad, copy the complete contents, and paste it here inside of 'code' tags. We'll need that to fix the underlying problem.
Just dropping to 720p is not likely to help, despite what your viewers have said.