Hi, i've been testing the net. Looking at statistics (MRTG, switch own statistics, etc) it doesn't seem to be any problem. This is a 100 1300 bytes packets ping:

100 packets transmitted, 100 received, 0% loss, time 99196ms
rtt min/avg/max/mdev = 1.758/2.080/3.664/0.403 ms

Actually i don't have the problem anymore, the last two days MRTG is working ok.

Thanks!