Hmm.. See if you can do the following:
Log in using IE and see if it happens.
Log in using Firefox and see if it happens.
Try a different computer (same internet connection) and retry the above (browsers)
Try a different internet connection (than you have tried so far) and see what happens if you use firefox & IE
lastly try making a new member account. See if the new account gives issues

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The above steps will greatly help me debug it. Also please go to this site:
http://whatismyip.com/ and see if over the course of the day your IP is changing

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