on Linux: strange behavior of cliente nfs with soft option

I have the following entry in fstab file
nfsserver:/usr/exec /usr/exec_test nfs soft,bg,noac,timeo=2,retrans=2 0 0

I need the cliente asnwer a I/O error message in, at most, 10 seconds, because i would try by another net.
If I take off the ethernet cable and try to acess the filesystem, the first time it answer in 2-5 seconds (OK). But at the second (third, ...) time it answer in 60 seconds.

Can someone help me?