hi stephane,

don´t know if you allready managed that, (please don´t call me zombie trhead maker..... ok you can call me ;)
sometime before I was with a problem like that, set statistcs solved it. only to have sure, you can backup restore.
if this don´t solve, maybe you can isolate this tables and send us an example of data to test (maybe it is a new issue ;)
Maybe I just telling what you already did (or know).

anyway, just trying to help :oD

bye