Hi Celine,
There are several topics about this fact:
http://forums.ni.com/ni/board/message?board.id=140&message.id=16871&requireLogin=False
also about NI-SPY and the ways to reproduce its "good influence" over your GPIB program:
http://forums.ni.com/ni/board/message?board.id=170&message.id=29823&requireLogin=False
Regard,
BRAUD Jean-Philippe
Field Sales Engineer - Nord et Centre France
LabVIEW Certified Developper