Have you just upgraded PHP? Have you tried increasing memory_limit in php.ini, restarting PHP and using phpinfo(); to double check your memory increase worked?
I've seen so much [expletive] code in magento and people think it's a PHP setting issue because extension providers tell them "it's your server".
It's bad code.
Magento can run in ~50MB of memory, maybe 100MB if you've got massive catalog pages.
Bad code causes these things in my experience
So gotta find what/where the crappy extension or code and kill it The way to do this is to enable the Magento Profiler. It tells you how much memory is being used by each layout block which can be helpful.
To enable it update index.php and uncomment the profiler lines then go to your Magento admin System --> Configuration --> Developer --> Profiler and enable it. It will appear at the bottom of every page. If this is a live store add your IP to the "Developer Ip Filter" textbox above on the same page
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