Well, after much testing I've found that the problem is the pcn(4) network driver. As I told you in the dmesg I was using VMware, specifically the ESX.
I found that when I used heavily the network (such as a long "ls" from the shell), the output just freezes even with a listing of 1000 directories. # cd /mnt/remote # ls freezes after a while # cd /mnt/remote # ls | wc works in every case, no matter how much directories I have in the remote. So it seems that this driver "just works" but is not usable with heavy network traffic. I set up a physical machine, identical to the virtual but with the fxp(4) driver and everything works ok, even the "ls" of 20.000 directories. I found some other posts here recommending to use the le(4) driver or the vic(4) driver in the virtual machines, but I'm not sure of the troughput I will get with those drivers in case they work ok (I see posts doubting it). So the decision is switch to a physical machine and experiment sometime later with a virtual machine in the ESX server. Thanks to all of you for your aid in debugging. -Joaquin Herrero