Endless widget save and menu save loading

Hi, I own a wordpress multisite blog with a lot of plugins actives on a shared web hosting. The problem is that I can’t save widgets and menus, an endless spinning wheel appears. PHP Memory limit in php.ini is set to 256 MB. I tried to change wordpress memory limit that is 64M to 256 or more but I couldn’t. I tried editing wp-config, .htaccess (in public_html and /wp-admin) and also creating php.ini in both folders but nothing happened. If I add memory limit line in .htaccess I get 500 Internal Server Error.
Help me please. Thank you in advance.