BibTeX, biblatex and biber ⇒ caption and citation with natbib and caption packages

[kmcfarlane]

I am using natbib with plainnat, and caption.
\usepackage[numbers]{natbib}
\usepackage[format=hang,hangindent=0pt]{caption}
.
.
.
\bibliographystyle{plainnat}
\bibliography{MS}

I have problems with citations in captions. This code works some of the time:
\begin{table}[ht]
\caption[Standard model fundamental particles]%
{Standard model fundamental particles~\protect\citep{PDG:2008}}

However, sometimes it breaks after I have edited files and gives me an 'undefined command' message pointing to that line. When it does break, I cannot 'heal' it. I have tried deleting all the aux files. Sometimes getting out of MikTeX and coming back in works.

Any ideas why this combination should be unreliable?

[sommerfee]

Could you please post a complete example? (*) Using this I would be able to reproduce and hopefully fix the problem.

Axel

(*) http://www.minimalbeispiel.de/mini-en.html