Sparodic 404 error

Hi

I keep getting sparodic 404 errors on the site.

Prime example is: http://blogsmu.com/wp-admin/

We've only had the site up for a few days.

Everything appeared to be working ok until we started installing the plugins from here (no blame being apportioned).

Before you ask, yes we did test between plugin installs, and everything appeared fine.

I don't like the prospect of having to reinstall again.

I go to the page and it works, then it shows a 404 error, then it works, then it works for a couple of times then it shows a 404 error again. This also works when you just press f5 (refresh).

It also happens to other people who use the blogs too. It throws up random sparodic 404 messages all over the place.

No matter what page it is, it still shows the url correctly and I know they exist.

I don't know where to start. Typing 404 error into either WP MU support or here brings up too many posts and I will be here forever.

Please help, it is doing my head in.