Google, Yahoo, and Apple do not load, but most other sites do

[sfromhart]

On my one of my iMacs, any site of Google, Yahoo, and Apple (including their search engines) will not open as well as a few random websites. Most other websites open immediately. This is happening exactly the same on both Firefox and Safari, so it does not appear to be a browser-specific issue. The operating system is 10.6.8. The other iMac in the house (OS 10.7.5) is working fine as well as our Macbook, iPad, and three iPhones. I mention the other devices because I believe the problem started after we changed the Verizon modem and wi-fi network, but I cannot be sure, and I don't understand why the other devices/machines would not also be effected. There is no spyware that I know of installed on this problem machine.
What I've done so far:

• Cleared all history and reset both browsers.
• Ran a "repair disk permissions" on the Utility Disk and received many messages that were variations of the following: Warning:SUID file"System/Library/CoreServices/RemoteManagement/ARDAgent.app/Contens/MacOs/Ar dAgent" has been modified and will not be repaired.
• Cleared the caches in my hard drive Library and Users Library folders (the Internet is now faster for the sites that do work)

 

[shokunin]

Check your mac for malware. Yes, malware. Check your DNS settings under networking to make sure you don't have a rogue DNS server listed. I think there are websites that can check if you have the DNS Changer malware on your mac.

 

[sfromhart]

Checking for Malware on iMac

Thanks, Shokunin. Does anybody have suggestions on how I check this Mac for malware? Any insights on these websites that Shokunin believes may exist that could check for the DNS Changer malware?

 

[sfromhart]

Still Unresolved

Wow, this forum has proved most unhelpful. Is this really all the response that I'm getting in response to my thread? One responder telling me to check for malware, with no specifics on how to do it?
Gee, thanks, Mac Rumors.

 

[ClamshellOfDoom]

Check your mac for malware. Yes, malware. Check your DNS settings under networking to make sure you don't have a rogue DNS server listed. I think there are websites that can check if you have the DNS Changer malware on your mac.

Here is a link to the tool that removes this trojan.