PHP-CGI Started Serving Blank Pages

Supercache generates full cached versions of pages, as well as doing some other internal caching, it's possible there's some sort of timing issue in high-load situations. I'd check your cache folder to make sure it doesn't contain blank files. If it does, you probably have a file locking issue. Solving such an issue may be tricky, and will probably require you to open a bug ticket, as it's likely a problem with the code itself.

have you tried actually stopping the fcgi service and starting it again as opposed to restarting it. i have had similar weird things happen while restarting fcgi

Check the response status of your requests if it happens again (eg. curl -i) and if the status isn't 200, check your error pages (specifically the one(s) for the error number shown) and ensure they aren't set to redirect in a way which might cause an infinite loop.

If the response status is 200, check whether there's any text in the response other than the HTTP headers.