It's an even Debian Lenny problem inherited by ubuntu in multiverse/uniberse.
http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=497572
Try:
Untill fusesmb newest version is fixed:
In the manual don't use fusesmb. install smbnetfs package instead.
All the other directives are fine.
update:
workes longer, fails with same symptoms eventually.
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