Weird forum glitch

Recently, I’ll go to click on a thread or even a forum category and instead of loading it, I’ll get one of those dialogue boxes asking if I want to open such and such a file. Depending on what I click on, it will either be ‘reply.php’ when I click on a thread and when I click on reply, and when I click on a forum category it will be 'forumdisplay.php. If needed, I’ll take a screenshot next time it happens. Only happens on this forum.

Oh yeah, using Firefox.

It happens when the server is overloaded and can’t process any more php requests