I frequently the below message when using Google search:
Our systems have detected unusual traffic from your computer network. This page checks to see if it's really you sending the requests, and not a robot. Why did this happen?
This page appears when Google automatically detects requests coming from your computer network which appear to be in violation of the Terms of Service. The block will expire shortly after those requests stop. In the meantime, solving the above CAPTCHA will let you continue to use our services.
This traffic may have been sent by malicious software, a browser plug-in, or a script that sends automated requests. If you share your network connection, ask your administrator for help — a different computer using the same IP address may be responsible. Learn more
Sometimes you may be asked to solve the CAPTCHA if you are using advanced terms that robots are known to use, or sending requests very quickly.
Then I have to solve a captcha which I normally get incorrect and then have to solve another one. I don't have any malware or suspicious plugins. It seems to frequently happen during peak hour, both in Windows and OS X using Firefox.
I think the problem is to do with my ISP using a shared IP for all users. I have filed a support request with Google without any luck. Any other suggestions?