At some point in the last year or so my Mathematica OSX installation has developed font-rendering problems that make it basically unusable:

The text of In[3]
is:
Simplify[(2*x*a*S*C + b*C + S)^2 - 4*a*C^2*(x^2*a*S^2 + x*(b*S - C) + c)]
I excerpted the error dialog as well in case that's relevant.
Anyone seen this or know what I can do? Thanks!