Marco,

I've encountered the same behaviour and the problem was due to the network interface which misses some packets from time to time. I was using a National DP83846 as the PHY and the clock accuracy (+/- 25 ppm needed) wasn't good enough.

Are you using a PHY from national?

Laurent

P.S : By the way, Wolfgang is right. It's not an u-boot topic.

Laurent MOHIN ACTERNA