Originally Posted by WiredGuy

Its very common for extremely precise query strings and for frequent searches. If you're getting into a loop where you keep getting denied even though you think your entering the captcha perfectly, try opening a different browser and trying again. I don't know why, but it seems to work. Might be a local cookie thats temporarily banned you. I've had that happen too.

Edit: By different browser, I mean a different brand browser like Opera or Chrome instead of IE/Firefox. Not just another session.
WG