Slow internet loading on new iMac

Hi there, i recently purchased a new 2GHZ G5 iMac 20" 512MB Ram and am having problems with the speed an internet page loads. My current setup is as follows.
We have a 3Mb cable connection which is connected to 2 routers which splits the connection to an Xbox, 2 PCs and my mac. The mac connection enters a hub which allows my old mac (Powermac Dual G4 450 448MB Ram, OS 10.3.6) and my new iMac to connect to the net.
Ok, so the problem.. On the Powermac, the internet loads very quickly on both Firefox and Safari. However on the iMac, the loading of any page is staggered and slow. It seems to spew out some of the page very quickly then pauses, seemingly idle, then proceeds to spit out the rest of the page. The imac is approx 10 secs slower on both Firefox and safari.
Both macs connect via built in ethernet using DHCP.
I have tried swapping the ethernet connections, however the results are the sam, powermac is always fast and imac slow.
I have tried connecting directly to the Cable modem, again no joy. An upgrade to 10.4.1 didnt help either.
So it seems that despite the quickness of the internet connection the mac seem to connect quickly, pause, the proceeds.
Is this a hardware problem with my built in ethernet, 10.4, or is there just a simple box that needs unchecking in my network preferences. Please help