What is it?
During the game blocks will reappear after being destroyed, or disappear after being placed for a few seconds before doing what they're supposed to be doing. Block lag, shaky sun, and shaky mobs are all side-effects of this problem.

What causes it?
Hostile mobs are always trying to spawn. They have nowhere to spawn during the day because it is too bright on the surface of the islands and there are no caves. But they keep trying. That's what causes the lag, hostile mobs trying to spawn but not finding anywhere they can. This is a Minecraft issue.

What does the map do about it?
We set the game to peaceful during the day, the only time where there is no place for mobs to spawn, so they stop trying. For whatever reason your game may not turn peaceful or the problem will persist after it is peaceful. To see if the game is in peaceful mode, run around. If your hunger bar shakes but doesn't deplete you are in peaceful.

I'm in peaceful, but it still happens!
There are still many things we do not know about this issue. Processor and computer power may be a factor. If it's still happening, it may be because your computer cannot handle the map.

I'm not in peaceful, but I should be!
During the day you should be in peaceful mode. If you've used the test above and you are sure you are not in peaceful mode during the day, please tell us! Include your method of playing (Singleplayer, multiplayer, Bukkit, etc.) and if the solution is fixed after the first day of gameplay. Bukkit is not supported until they get the place block command. If you want a faster solution, try completely restarting the game. Download the latest version and play again.

If you have any more questions, please post below.