font change while using an application

Hello everyone,

since I set up Hardy Heron on my computer I've problems with this strange bug: While using any application (Firefox, Pidgin, Geany, Thunderbird) it happens, that the font mysteriously changes while I'm typing. In the most application where this error occured I had to close the whole application to rid of the font, Geany (Texteditor) as an exception works when I just close the tab an opened file and reopen it.

I couldn't find any pattern or typical circumstances when this error occurs, it's just when I'm typing. I thought of how to report this "bug" and began to make Screenshots whenever it occurred. I hope you can see some pattern or the cause of this problem...

Greetings from Switzerland,

Dominique

Ubuntu Hardy Heron 8.04, GNOME