Thanks for the suggestions everyone. I've got to the bottom of the problem now (I'm sure there will be a collective sigh of relief from the list because of the noise this thread generated :-)). I installed two brand new, low spec, 3Com switches one at the 'front' of the network and one 'behind' the routers. They are the same model, same latest firmware, same config (saved to and then copied off disk) and only their IPs were different. The front switch was the problem. As two final tests before removing it we switched off unicast/multicast broadcast control and flow control (and simultaneously on the same port on the switch behind the routers) because there were pause frames showing but not a massive amount in terms of percentage. The switch behind the routers however is serving the same bandwidth equally well ! We've put an ancient switch in place of the front switch and its been working perfectly so far. My lesson learned is too change as little as possible at once ! That said, recent network changes were spread about a month apart and this very odd issue was far easier to dismiss than believe due to its bizarre nature. Especially when providers have changes in their network conditions as testing is done. I really appreciate all the input and have learnt loads, possibly just not in the way I would have liked to :-) Doubles all round, Chris