Help please!

Hello sometimes this error apears me in my forum and i don´t now why because it apears not every time i try to go to the arcade mod but sometimes, usualy after i make an optimization of my tables in the acp admin it always give me the error...

Fatal error: Allowed memory size of 8388608 bytes exhausted (tried to allocate 737280 bytes) in /home/sites/ghostteam.org/public_html/red/arcade/functions/functions.php(1187) : eval()'d code on line 498

Can someone please could tel me why it apeans?

Thanks to you all.

Kind regards.