[XeTeX] refusal to typeset and strange behaviour

[Alex Hamann]

I am working on a paper that is saved in Utf-8 format with this preamble

\documentclass[12pt, oneside]{article}
\pagestyle{plain}
\usepackage{setspace}
\usepackage[applemac]{inputenc}
\usepackage[left=25mm,top=25mm,bottom=2cm,right=25mm]{geometry}
\usepackage[marginal]{footmisc}
\usepackage{fontspec}
\newfontinstance\fonta{Gentium}
%\newcommand\basetext{\fonta\fontsize{12}{14}\selectfont}
\setromanfont[Scale=1.01]{Gentium}
\usepackage{ucs}
\usepackage{sectsty}
\usepackage{url}
\usepackage[sort&compress]{natbib}
\bibpunct{(}{)}{,}{a}{:}{,}
\bibliographystyle{plainnat}
\citestyle{}
\begin{document}

the text contains Greek letters. When I include Greek fonts they are  
properly displayed and typset. However, as soon as I save, close, and  
re-open the file, the Greek fonts are relpaced by apparently random  
symbols (or maybe it is the unicode fonts in another encoding? lokks  
like this:  γεη σψηον  ). It still usually typsets the  
correct Greek fonts, this morning, however, even though I had made no  
changes to the file since the last successful typesetting, tex  
refused to typeset with the following report:


! Missing $ inserted.
<inserted text>
                 $
l.29 ...note{γεη σψ ηον}
                                                    in the beginning  
were als...

?


Also, this morning XeLaTex refused to correctly display ALL Greek  
fonts, event those I included in a minimalist tex-file and tested  
right away. Strangely, now, after having done anythin than just  
repeatedly trying, it works again. (Yes, I know how this sounds...)  
In this case the Greek fonts were just replaced by "?" both in the  
pdf and in the tex file after that one had been saved and reopened.

What is going on?


TIA,
Alex