I use a modified "Shape" theme for my WordPress. It has a custom 404.php page in its folder. I wrote some stuff there, like "sorry page wasn't found, you can try searching" and placed a search box so it would look nicer than an ugly default 404 page that browsers show.
Here's the problem though. In Firefox and Chrome the custom 404 page displays just fine. But IE8 insists of showing its own default error page instead (the "The webpage cannot be found" one).
I checked the site headers and it sends a status code "HTTP/1.1 404 Not Found" properly.
Here's a really weird thing though. When I attempt to open a non-existing URL I made up in IE8, maybe 9 out of 10 times it displays the default 404 page. But once in a while it shows my custom WordPress 404 error page. It's completely random!
Any ideas what could cause this?