Good evening Daniel,
First, thank you for the answer. I actually managed, in the end, solving that problem : i exported the videos directly from Avid, in DV PAL (lower field, then) directly, probably not the best option, but the only one that worked so far, as the deadline was closely coming. Then we let Encore do his thing, and no jittering issues anymore.
As you said, I did make a mistake. So far, I always thought that SD format was in DV PAL, but I painfully understood that it was only the other way around. Thinking back about it now, what I thought was actually pretty stupid.
Anyway, thanks a lot for your answer.
Bye,
François