One OS X imac not accessing some HTTPS sites while using Xfinity but can while using Verizon.

Strange problem started happening on my home network.  Have 2 imacs running OS X 10.5.  
One of them suddenly will not open some HTTPS sites like one of my email sites.  I can open this email as well as all other HTTPS on the other imac on the same network.  My ISP is Xfinity/Comcast.  i tried connecting by ethernet to the Xfinity Gateway and still no access.  I tried creating a wireless Hotspot using a different ISP (Verizon) and Viola! I can connect.  I go back to the Comcast wireless connection and no connection again.  This is happening on both browsers on this computer.  Tried clearing DNS via Terminal.  Tries swapping out the Gateway (though only one mac is affected. still same issue). So something with just this mac and Xfinity?  Xfinity cannot figure since it works on the other mac.
cdent99Asked:
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cdent99Author Commented:
Needed to 'clean' install to resolve this issue.
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Seaton007Commented:
Check to make sure parental controls aren't enabled on the mac that's having problems connecting to HTTPS.
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cdent99Author Commented:
Parental Controls Not enabled.  Can access the HTTPS sites if connected to a different ISP.
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XaelianCommented:
Can you take a look at the advanced settings of all your network connections?
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cdent99Author Commented:
Took a look at advanced settings...I can access all HTTPS when on a verizon or likely any other ISP.  Just this particular computer is doing this. And only when connected to the internet using Xfinity.  But the other mac, also on Xfinity, can access the sites.
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cdent99Author Commented:
No other solution found but the ultimate clean installation
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