Frames problem

Anyone got a quick answer for a novice dreamweaver user?
I have made a site for a client and the buttons (situated in a top frame) are rollover images that also instruct the mainframe to change to five different pages.

When I click on one link the transition is smooth.

When I click on any other link the mainframe image is flashing before appearing (it is the same jpeg for each new page).

I cant understand why it doesnt for one page but does for all the others ?

**** annoying and irritating to look at!

HEEEEEEELP!!!

http://www.cozmek.com/priory/index.htm for anyone who wants a look and can give me some advice.