Slower internet on only one computer when BT HH3 swapped out

Hey guys. Long time lurker, first time poster

I have BT Infinity Option 2 and I'm lucky enough to get the full 80/20. When using the BT Home Hub, all the wired computers download at around 75-80Mbit/s. When I replace the Home Hub with a Cisco/Linksys WRT320N running DD-WRT one of the computers drops to 60Mbit/s. The computer that has this problem is a newly built Hackintosh, dual booting Windows 7 and OS X 10.8. The problem is identical in both OSs. All cabling checked, as well as trying a different Ethernet port on the router. What the hell could be causing this? All I can think of is that the WRT320N doesn't like the HP NC360T Dual Port Gigabit NIC in the affected machine. But why doesn't it have a problem with the Home Hub?

tl;dr: Internet fine on all computers when using Home Hub, slower on only one computer when swapped for what should be a better router.

"interoperability" - not all chipsets and devices interpret standards the same way, so differences can arise. More common in woollier standards like wireless than wired.

Do the ports still report Gigabit connections ? Do you see a difference in LAN transfer rates between machines ?

Router LEDs indicate that all active ports are running at Gigabit speeds. Adapter status window in Windows confirms this. LAN transfer speeds are fine. I'm getting 80-85MB/s between the affected machine and the others on the network.

Some more info; I just re-activated the on board LAN controller to see if that made a difference (It doesn't support WoL under OS X, hence the PCI-e NIC) and I get exactly the same problem. All be it my Internet speed jumps to 62Mbit/s. When I swap back to the Home Hub, bam, back to 80Mbit/s.

These problems occur ...

I have four "identical" routers but two of build standard -008 and two -009. If I run the -008 through the modem I would get a stable 36 Mbps but changing to the -009 results in speeds varying between 6 and 30 Mbps. I, BT and 2ire have not found the cause!