Fontification Problems with alltt

Dear AucTeXers,

while writing a LaTeX file with frequently interspersed code
fragments typeset in the alltt environment, I experience that
AucTeX has a tendency to somehow get confused about whether it's
"inside" or "outside" an alltt environment. The visual effect of
this is that everything is fontified as though it was in alltt,
except the portions inside an alltt environment.

Switching font-lock-mode off and on again fixes the problem, but
it eventually returns.

Unfortunately, I am not able to systematically reproduce this
effect. I have the impression that the chances of this happening
get higher as I work more on the end of the text, but it occasionally
happens even with very short documents.

This happens on emacs-21.3-12, running on a Fedora core 2 installation,
but I've also seen it on my notebook which is a recently installed
Debian system.

Is this a known problem, is there anything that can be done about this?

Kind regards,
Jan T. Kim