Lag seems to be random from what I can tell, although there will be some common reason for it, deep down somewhere hard for me to find... If you notice lag, or a server crash, ask everyone what they were doing and report it in detail here, along with everyone who was online. Hopefully then we can find a common problem. But like ako said, make sure it is not client lag, only server. This includes block lag, players "flying" while jumping, and commands / chat taking a bit to register.