[OS X Emacs] .Rnw files in Aquamacs font problem

[Caroline Wiley]

I have changed my default font and size to Monospace 14. I have been using
Aquamacs for a very long time and only with the past 1.7 upgrade have I had a
problem with the default font I set. In .Rnw files the font size will
sporadically change to 13 then back to 14. This makes it very inconvenient for
scrolling. The font is stable for all other file types (.R, .tex) and only seems
to be an issue with .Rnw. Any suggestions?