The first thing to try is manually deleting the files in the core/cache directory.
It could be a site hacking attempt. Check for signs of tampering in the index.php file in the root.
You might also check your templates for foreign stuff at the top.
Disabling any plugins that operate in the front end (e.g., tied to OnWebPagePrerender similar events) is something else to try.
You can also put a space between the first two brackets of any snippets in your template to disable them.