Problem with correct site loading

I have a frequent problem with wpmu not selecting the right site to load.

For example, if I navigate to my main site, wpmu "may" load a subdomain site. I have not noticed it loading the main site if I go to a subdomain.

Moments ago I clicked on the bookmark to go to the site domain and a subdomain loaded. I clicked refresh and the main site loaded correctly.

It does not happen all the time.

Any suggestions?