sometimes apache dishes out text/plain

I'm seeing some very bizarre behavior from (I assume) my apache2
server. I have 2 PCs on a local network. I use one for development and
the other to test the web pages I make. Both run Mandrake linux. The
development machine is running apache2 (v 2.0.48).
Apparently at random, for the same web page, sometimes apache dishes
out text/plain (so I see just raw html text in the browser), and
sometimes it dishes out text/html so that it gets digested properly by
the browser. I've tried mozilla and opera with the same results.

The web pages all have the meta tag:
<meta http-equiv="Content-Type" content="text/html;
charset=iso-8859-1">
This happens even if I carve out the body of the page leaving only the
header. So it's not the html.
Rebooting (and hence restarting the server) doesn't help.

I know apache dishes out text/plain when it doesn't know what the
filetype is. But this is html. What's more, it does this at random for
the same page!

What on earth could cause this??

Thanks.