Allowed memory size exhausted

Archiving period = year for idsite = 1...
Fatal error: Allowed memory size of 268435456 bytes exhausted (tried to allocate 4104 bytes) in /var/www/piwik/core/DataTable.php on line 935

During the installation I had set the memory_limit to 32M.

On 11 April, I had the first problem with “Allowed memory size”. After that I had increased the limit every Sunday:

  1. april
    from memory_limit = 64M
    to memory_limit = 128M

  2. april
    from memory_limit = 128M
    to memory_limit = 256M

Today I have the same problem with a memory_limit of 256M again. What can I do now?

I currently use Piwik 0.6rc2