Try (if you already have not tried):
-deactivating ALL plugins temporarily to narrow down and possibly fix the problem. If the problem goes away, activate them individually to find the culprit.
-switching to the default theme ("twentyten") for a moment by renaming your current theme's folder in wp-content/themes. The idea is to force WordPress to fall back to the default theme to rule out any theme-specific issue.
-If the above steps do not resolve the issue, Download WordPress again and replace your copies of everything EXCEPT the wp-config.php file and the /wp-content/ directory with fresh copies. This should replace your core files without changing your content and settings in wp-config.php file and the /wp-content/ directory.