Language input settings keep switching

Well, assuming you're using Tiger, a few things you might double check:

In the Input Menu section of the International pref pane, make sure that the "select next" and "select previous" key combinations aren't set to something that you also use in, say, photoshop.

For example, I prefer the old-style command-space shortcut (and use it constantly), but in 10.4 that defaults to Spotlight, so I need to change the Spotlight setting to something else (it does warn you in this case, but not if it's the same as a Photoshop command).

The other thing to check would be the Input Source Options section of that prefpane; I've found that if you have "Allow a different input source for each document" selected, it has a tendency to get "stuck" in one input source and keep switching to it whenever the focus changes. Using "Use one input source..." clears it up, though.

Well, almost... I have occasionally seen an issue where Camino keeps changing the keyboard when I click on its location bar, but that's erratic and uncommon.