@kishore:
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Did you check the cables, switches, and other external stuff such as routers, repeaters etc? Maybe something is running in half duplex or a swith has limitations, uses 10Mb or is overloaded etc? I would look outside your firewall first since your hardware is clearly capable. Also If you are using old network cards or brands such as realtek (not Intel) you may want to check that these work correctly and that you have a good and working driver.
Some network cards use other manufacturers circuits (typically realtek) and may have problems, I am not saying that no other NICs than Intel work or work good (they do!) but I have ran into wierd problems a couple of times when the cards did not "work" properly. There are some threats about some of these issues I believe. If you can, try to replace a card for an Intel card (the cheapest desktop pci-cards for example, these work good and have a long life time) and see if the problem is still there. Sometimes you need to fiddle with the drivers and parameters. Did you google the card name?
/E