Thanks loads for the quick replies. I'll try and respond individually. Lee > I recently disabled tcp_window_scaling and it didn't solve the problem. I don't know enough about it. Should I enable it again ? Settings differing from defaults are copied in my first post. Mike > Strangely I'm not seeing any errors on either the ingress or egress NICs: RX packets:3371200609 errors:0 dropped:0 overruns:0 frame:0 TX packets:3412500706 errors:0 dropped:0 overruns:0 carrier:0 The only errors I see anywhere are similar on both NICs. Both connect to the same model of switch with the same default config: rx_long_byte_count: 1396158525465 rx_csum_offload_good: 3341342496 rx_csum_offload_errors: 89459 and it may be worth noting that flow control is on. Are these a reasonable level of pause frames to be seeing ? They seem to be higher on non-routing boxes. Total bytes (TX)2466202288Unicast packets (TX)3436389971Multicast packets (TX)213310Broadcast packets (TX)4952902Single Collision Frames (TX)0Late Collisions (TX)0Excessive Collisions (TX)0Transmitted Pause Frames (TX)27806 Florian > They're running without obvious errors. Auto Neg has taken 1Gbps, Full. Can Auto Neg cause these symptoms do you think ? Thanks again, Chris