Accented characters sometimes work, sometimes don't

[John Cook]

I have a webpage with lots of accented characters from different languages. In IE8, sometimes the page loads fine but sometimes selected accented characters (i.e. á, í, é, ð) come up as boxes. It seems completely random whether it works or not - just reloading the page without making any changes can show different results.

In my HTML header, I use

<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />

At the start of the PHP code, I use

header('Content-type: text/html; charset=utf-8'); 

Any tips on why it's intermittently working like this?