Internet & Ichat Inconsistent


Hello, I sould use some help troubleshooting my:
iMac 17", USB2
512 ram

Over the course of time (last two weeks) my internet has slowed down, recently ichat & firefox have become unstable. (same story applies to IE & Safari) Sometimes connecting, albeit slowly, somtimes not connecting at all.

Up until my attempts to troubleshoot today, my PC notebook worked fine...AIM & my internet usage was unhindered. Today, AIM is ok, but internet service is spotty at best.

I have used mac janitor & onyx to clean up cashes, etc. I turned off Rendezvous as well, in terminal. I have run disk utility from the start up disk to repair permissions. The firewall on my Netgear router is off and the firewall on the iMac are off...both previously on when things were working fine.

Adelphia, my cable service, tells me at slightly above average speeds. And course I have repeatedly tried cutting the power from the cable modem & router along with rebooting the mac.

I need help...any tips out there?