Problem with natbib and plainnat

[Hassoun Karam]

Hi
I am using a custom class (developped by my university) to write my 
thesis. It inherits the report.cls. When I try to use the following 
configuration for my bibliography (see the tex export code)  I get the 
error:

-----
TeX capacity exceeded, sorry [input stack size=3000].
 \begin{thebibliography}{1}
If you really absolutely need more capacity,
you can ask a wizard to enlarge me.
-----
My .bib file follows the bib format. I have also included below the 
bibliography code from my custom class. When I use another style 
(Apalike, without Natbib) everything goes well, but I would like to use 
natbib.

any hints?
thanks a lot
Hassoun
ps. running Lyx 1.3.4 on MacOSX 1.2.8
%%%%%%%%%%%% the tex file %%%%%%%%%%%%%%%%
%% LyX 1.3 created this file.  For more info, see http://www.lyx.org/.
%% Do not edit unless you really know what you are doing.
\documentclass[frenchb]{theseUL}
\usepackage[T1]{fontenc}
\usepackage[latin1]{inputenc}
\setcounter{secnumdepth}{3}
\setcounter{tocdepth}{3}
\usepackage[authoryear]{natbib}
\makeatletter
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% User specified LaTeX commands.
[EMAIL PROTECTED] }
\usepackage{babel}
\makeatother
\begin{document}
test \citet{Gettys90} test test, \bibliographystyle{plainnat}
\bibliography{/Users/myself/bib_files/test}
\end{document}
%%%%%%%%%%%% the bibliography code from my class %%%%%%%%%%%%%%%%
\renewenvironment{thebibliography}[1]
  {\chapter*{\refname}%
   [EMAIL PROTECTED]@[EMAIL PROTECTED]
     [EMAIL PROTECTED]
      \leftmargin\labelwidth
      \advance\leftmargin\labelsep
      [EMAIL PROTECTED]@code
      \usecounter{enumiv}%
      [EMAIL PROTECTED]@empty
      [EMAIL PROTECTED]@enumiv}}%
   \sloppy
   \clubpenalty4000
   [EMAIL PROTECTED] \clubpenalty
   \widowpenalty4000%
   [EMAIL PROTECTED]
  [EMAIL PROTECTED]
    [EMAIL PROTECTED]@warning{Empty `thebibliography' environment}}%
   \endlist}