Special characters causing text to disappear

I’m having an unusual problem and I can’t seem to figure out how to solve it.

This is my first time using WordPress on my site, having successfully used it as a member on other sites.

Whenever certain special characters appear in my posts or pages, WordPress seems to stop all displaying content from that point onwards.

The characters I’ve identified so far are ? ￡ and any letters with diacritical marks, and also any curly quotes or apostrophes.

I can get around it by using the HTML codes, eg & copy ; instead of ?, but if I change from the HTML editor to the visual editor then back, the HTML code has been convereted into the actual character, which means when I update that the text ends at the point the character appears.

Any ideas why this happens, and more importantly how to stop it happening?