[OS X TeX] Intermingling fonts using Fontspec

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Greetings all,

First off, thanks for the quick response to my earlier question about 
speeding up rendering.  I've got a new question that's been baffling me 
regarding switching between fonts.  I think I'm doing something wrong, 
but I can't seem to figure out how to make things work right.

My issue is that I need to be able to switch between a standard serif 
font (in this case, I'm using Times New Roman) for the body of my work, 
and the Greek font Porson, which I use to typeset Greek words and 
citations.  I chose Porson because of its superior readability in longer 
passages.

To switch fonts, I declared a command \gk{} to use the Fontspec package 
as follows:

\newcommand{\gk}[1]{\fontspec{Porson}#1}

XeTeX (LaTeX?) seems to be smart enough to switch back to my standard 
roman font after the end of the final brace.  However, there are some 
strange effects that occur after I've included my first Greek citation 
in the text.  In particular, the Unicode combined double quote mark 
disappears, as does the opening single-quote mark.  Also, if the 
sentence as a whole is in a section heading, the heading begins in bold, 
but after the Greek it continues in normal weight.  Also, the FreeSerif 
font seems to suddenly turn on ligatures (although I'm not sure that 
Greek is the cause).

There must be some way to tell XeTeX to switch on Porson Greek for a 
certain selection of text, and then revert back to the standard Roman 
font, without losing the font features of the overall text group, or 
causing weird font effects to appear in the following text.

All help greatly appreciated in advance.

Saludos
Erik Norvelle

Universidad de Navarra,
Pamplona, Spain

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