Random error on a word document

[bobo]

Only on a big document (1.2Mb) make with office 97 and modify with office xp
and 2003 I get random error like this :
"word as enchountered an error and need ot close ...."
on debug i see that appname is winword.exe , modver is 5.1.2600.1217 and
modname is ntdll.dll.
I see this kind of error even if I hold it open without to modify it after
some minute but always random ....

Any ideas
Thanks
Bobo