[h-e-w] Undo problems in emacs 21.1

[Geert Ribbers]

Hi.

In some modes we generate / refresh a list in the upper part of files within
the local-write-file-hooks (so on every save of the buffer).
This works perfectly in 20.71 but in 21.1 it messes up the undo.
Until saving the buffer undo is perfectly, after save it is messed up and
it's not possible to undo more than a (very) few changes.
It looks like buffer-undo-list is messed up.
Has anyone got a tip for me?
This is a serious problem I think, undo should be reliable.

Thanks, Geert Ribbers