IMac connect to wi-fi but no internet but my other devices work fine

Question

I need some advice. I have searched the helps forum before posting so please advice what I need to do. I have tried some of the solutions from the forums but not working.
My scnario...
iMac (latest version) was working real good until 2 nights ago. It still connects to the wireless router and once a while get internet but 98% of the time, impossible to load anything. It will load very quick after reboot the system then it slows down to eventually dead....and then several minutes, it loads very fast for few seconds and went dead and repeat the same. I follow the network diagnostic to turn off the router, wait and turn back on....same issue. Then, I thought it might be virus or melware cause I downloaded some Songs from unknown site. I then installed virus and malware remover but still not working. I then tried to login to my router to check out but I cannot even login cause it will even get to the login page. So I thought router is bad but it's not. My wife's iMac, PC laptop, Apple TV, iPad, and Android Phones are all able to connect with 30MB speed. I have tried to follow the DNS thing and still not working.
I use my iPad for AirPlay using same router so I am pretty sure the router is good and internet is good. So it only leave me with one thing, my iMac.
Any help is greatly appreciated.

Answer 1

Hello...I cannot say anything about the malware/virus question... BUT, I wonder if something got screwy with your Keychain. So I clicked on the Help in the menu at top of the screen, while I had system preferences open... There is a lot there to read.. I am in 10.7.4, so it may not exactly match what your machine says (do that search too, for precision).. but one thing that I think MIGHT be associated with your dilemma could be the following (did you set new passwords before when you did as BDAqua advised? That will do things...) the window I found:
Applications that use passwords require your permission to retrieve your password from Keychain Access. An application may present a dialog asking you to confirm access to your keychain. The message may contain several options:
Deny: Prevent the application from retrieving the password from your keychain. You need to enter the password manually.
Allow Once: Grant Access to your keychain this time only. You’ll be asked again the next time the application needs to access the password.
Always Allow: Allow the application to always retrieve the password without asking.
If you want to allow an application, such as Mail, to always access your keychain without asking for a password, select “Always Allow.”
Open Keychain Access, located in the Utilities folder in Launchpad.Open Keychain Access
If you don’t see keychains in the left column, choose View > Show Keychains.
Select a keychain, from the Keychains list. The application’s keychain would probably be in your login keychain.
Double-click the password in the Keychain window, and then click Access Control.
To allow all applications access to this password, select “Allow all applications to access this item.”
To control access to this password, select “Confirm before allowing access.”
To require the keychain password instead of the application password, select “Ask for Keychain password.”
To add individual applications that can access this password, click Add (+), and then navigate to the application you want to add.
Click Save Changes.