It is possible to wait for a later initiative number in a round to act (p 100, initiative).
Is it possible to wait for the highest number in a later round, or do you then roll initiative for the new round to see when you can act/start waiting again?
For example, you are an archer who wants to disrupt a spellcaster. The first round of combat you do nothing but wait. In the second round you want to disrupt a spell. Can you shoot as soon as the caster starts casting, or do you have to roll and beat his initiative?