why does my master slide change?

[Lou]

I have created a pres. that has a title master and a slide master. I have
also created two supplementary design templates to be used throught the pres.
I create a new slide in slide sorter view, apply one of my design templates
[to my selected slide only] and then my original slide master takes on the
layout of the new design template. I have enable multiple designs per
presentation and keep design when copying slides on in the advanced
preferences. All I want to do is retain my original master layout in
conjunction with the occassional slide that uses a different master.