Ross,
I have noticed that my iMac doesn't have Verdana so it was using Times
New Roman instead and I'm not sure why because I think Arial is the
default Font. Anyhow, I'm hoping that explains the random Font change.
If the problem persists:
ActiveCell.Font.Name = "Verdana"
could be included in the code whenever it changes the value of a Chord
cell.

I did a bit of experimenting to see what would happen if the code
changes the Font Name to one that is not on the machine. I was
expecting an error to occur.
ActiveCell.Font.Name = "Poo" did not cause an error! The appearance of
the font in the affected cell did not change, it didn't even change to
the default Font, and "Poo" appeared in the Font Name Box on the
toolbar at the top of the window, even though the machine does not have
a Font named Poo.

Ken Johnson