utf-8 encoding problem

Instead of single closing quotation marks (and other similar symbols), some posts show strange character strings such as a€?.

I am using W3 Total Cache, with a database collation of utf8_general_ci and a character encoding of UTF-8. The incorrect characters go away if I clear the disk cache and refresh, but they return after a little while.

I currently have the following in my wp-config.php:

define('DB_CHARSET', 'utf8');
define('DB_COLLATE', 'utf8_general_ci');

Commenting out the lines only caused more obvious encoding problems.