Sorry to ruin the party guys, but I've been trying the kernel versions thing with no good results. Seems to be a solution for some cases only. I updated to Ubuntu 13.04 with 72Mbps top connections. Upgraded manually to kernel 3.9.2 also with no luck (had to go back to 3.8 because ATI legacy drivers don't work in 3.9). In this last case, connections were back to 150Mbps. I guess its something just to try ut.

Thank you anyway for all your help!