Your D-Link 604 should be UPnP capable. See Connection problems? Check here first! as there's instructions for setting up the dlink604 for UPnP. Only if after setting up your firewall, etc. you have problems then consider manual port forwarding. I don't know what D-link is like for upnp. I haven't seen any reports so far.

Hi cpu usage? Make sure you have Java 1.5
Don't use the Resume/Find Sources function on all your incompletes or otherwise this will use a lot of LW, & your computer's resources & may affect your comp's communication with your router.

Try the above & see how you go. If you need help with manual port forwarding then ask. But tips for this are also included in the link above.