Hello Antoine
I suppose, you just already installed the NiDaq upgrade (and LV patch) in accordance with your current version of Labview 6.1, if you didn't, check out this:
http://digital.ni.com/softlib.nsf/websearch/50F49E43A22C0D2186256BF3006B9A96?opendocument&node=132070_US
I just recently finished some development using the same configuration that you have described, and i had excellent performance, instead the other versions of Labview, if you want, you can give me more details about your problem.
JFT
systems engineer
ETT Ltda
Valparaiso - Chile.