I'm not using anything special in the router.
The issue with the older modems (and I don't think we've heard of any problems with *CABLE* modems) is the firmware on the modem being mismatched with the firmware at the ISP's end. The DSL ISPs in particularly don't bother to update customer modems unless they get a complaint/request.
If you are using Wifi, try connecting by ethernet to the cable modem, using standard DHCP for your setup in the Oppo. If that works better then you may have poor reception or channel interference on your Wifi setup into the Oppo. Try moving the Oppo antenna (that's what the extender cable is for) and/or change output channels on your Wifi base station. If your router is separate from your modem, try bypassing the router. If that fixes the problem then perhaps you have firewall or other settings in the router that are slowing things down (although that should affect your PS3 the same way).