Redirect randomly failing in IE 8 and older
Does adding exit or die(); to the end of your PHP make a difference?
Sometimes HTTP clients don't like it when a redirect header is sent while content still exists in the body (even if it's empty content: spaces, line returns, etc.)
What happens if you use rawurldecode($lp)? I've tested in Windows 7 in IE7-10 and it works fine for me (both ways). Strange!
Check the URL you're being redirected to in IE, what's the address bar say?
To prevent problems with header redirects I usually use ob_start(); and ob_flush();
<?phpob_start(); //at the top of the page//header( $location ) ;ob_flush();?>