Wombat the same thing happened to me two times with my cable provider. My internet would be fine and during peak hours it would go extremely slow or disconnect. We tried calling to see if this could be resolved but it was always a pain. The first time the problem was resolved when a tornado took out all the telephone poles around my house. Last summer when this happened again it turns out the cable outside had water damage.
Good luck having them come to your house. It took a tornado and paying for a higher speed internet to get Charter to fix it.