Some of the PHP code can cause these types of problems, not the style. It's mainly MODS, PLUGINS, and CUSTOM CODED parts in your forum.
yes, i think the problems in the plugins, i'll disable all of them ...

1. Disable User's Online List
2. Install Xchace/WinChace.
3. DB Optimizer plugin's.
4. Different shoutbox.
1- i'll do this
2- what is this : Xchace/WinChace?
3- i'll do this manually, i don't have this plugin
4- i don't use shout box

thank you l0calh0st and VHostZone , i'll do what you tell me..