Lighting behaving badly...

I’m having strange problems with my light.
It changes position (or, the other stuff does) depending on what is drawn on the screen…
Briefly, this is what I do:
glLoadIdentity();
gluLookAt(XXX);
glLightfv(XXX);
glPushMatrix();
glTranslatef(XXX);
glRotatef(XXX);
do the drawing
glPopMatrix();
various stuff
glPushMatrix();
glTranslatef(XXX);
glRotatef(XXX);
do some more drawing
glPopMatrix()

and so on…
what might go wrong???