Intermittent Internet Weirdness

Our internet connection stop working intermittently, I'll be working along and all of a sudden web pages won't load and email won't download, the weirdness is that I can run Itunes and connect to the local NPR radio stream. The radio works fine when Firefox, Windows Live Mail, Internet Explorer, and Google Earth won't. We've replaced the cable modem and patch cable. There's no router, the PC is directly connected to the modem. HELP! Thanks.

Tech Support Guy System Info Utility version 1.0.0.2
OS Version: Microsoft Windows 7 Home Premium, Service Pack 1, 64 bit
Processor: Intel(R) Core(TM) i7-2600 CPU @ 3.40GHz, Intel64 Family 6 Model 42 Stepping 7
Processor Count: 8
RAM: 8173 Mb
Graphics Card: NVIDIA GeForce GT 530, -2048 Mb
Hard Drives: C: Total - 939341 MB, Free - 861710 MB;
Motherboard: ASUSTeK Computer INC., CM6850
Antivirus: Webroot SecureAnywhere, Updated and Enabled