I belive it's becuase we're running on a load balanced system... IE there are several web servers feeding the site. MaxPC and all Future Network properties are hosted by a company called Atomz
. They specialize is high-avalibility sites for big businesses and corporations.
as a result of what I was say is a safe guess it is clock drift between the servers. phpBB does a basic time check.
it takes the current server time and subtracts from it the time of your last post. if that time is less than the so called "flood time" it exits and throws the error.
The problem arises with the drift between servers. Lets say you make a post on server A and it sets your time for the post. Now you go to post again but this time server B is accessed. Well becuase it's clock is say a minute or two(or God forbid 10+ minutes) behind server A when phpBB does the time check it basically gets a negative value and will throw the error. Every time you try to post if you get server B you have to wait till it's clock catches up to the "fast" server plus the 15 seconds before it'll let you post.
The only way to remove the problem is to remove the flood protection or get the server admins to keep their server clocks synced better. This is surprisingly a simple thing to do yet too many admins never think to even do it. It's very important especially in load balancing situations.
Pointless but interesting fact: php and mysql use the unix timestamp for tracking times. Unix timestamp is the measure of seconds that have elapsed since January 1 1970 00:00:00 GMT. This is known as the Unix Epoch.