Okay here are my possible reasonings for this
- Your computer doesn't have enough RAM to support Flyff that well.
- Your computer can't handle the program for over a certain amount of hours
- The server was extremely packed, and the more players the more lag is produced.
- Your router fails and you need to reset it every once and a while.
Those all happen to me usually... so that's what I am assuming is wrong with yours.
And what kind of computer do you have? This can lead to a few problems as well.