After few minutes of work big packets are droped.
I tried kernel 2.6.15-1.2054_FC5 and 2.6.16-1.2122_FC5, but I have the same problems. When I restart network it is normalized, but over few minutes it coming back. I think this problem of packet fragmentation. Now I set limitation of MSS in iptables and don't have this problem, but I think it is not the good decision.