From: Andrew Hoffman on
We had a similar file at our organization. The issue seems to be (at least in our case) whomever created the file made it so that any new text created in a text box was set to be a "subscript". After a number of characters were typed, the font would suddenly revert and not be a subscript anymore. When all the text was selected and had the subscript format turned off, the issue went away.

Andrew Hoffman
Corgan Associates
Dallas, Texas

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frmsrcurl: http://msgroups.net/microsoft.public.powerpoint/Textbox-problem-text-changes-font-size-inside-the-box-randomly