squashed test in combobox
 

I have placed a combobox (from the controls toolbox, not forms) onto a
worksheet. When I select an entry from the dropdown list the selected text
in the combobox looks fine, until I move the focus away from the combobox (by
clicking on another area of thew worksheet, whereupon the text become much
larger and the letter are squashed together and overlap each other... almost
looking like a graphc rather than text.
If I select the combobox again the text is fine... until I move the focus
again when the text revertes to its "squashed" state.
Any idea why this is happening and what I need to do to prevent it happening?

Thanks,
Conor