because your internet is slow, unstable and constantly disconnecting!
I don't have these issues and I'm on Undernet many years. I have idle from 30 January 2018 and no disconnect was! I have an unwanted disconnect once-two per month. So you call this unstable?
Make sure you have a good internet and not shit one with package losses, also pay attention what Undernet server you use. I suggest you to use Budapest server.