Http error in Google webmaster tools, why?

Hi I am slightly confused and perplexed over an issue with Google webmaster tools. Two of my sites are showing the crawl error HTTP - domain name not found. However they are both searchable. What is also strange is that the faults were detected before the sites went live and have been regularily updated, what may be significant is that both domains were dropped previously.

Can someone please give answers to the following, why is Google constantly showing this error, is it or will it affect my rankings, and can this be resolved?

have you checked the robot logs to see what they have been up too, there would have to be something doing it. Also do you have a robots.txt in the index too. make sure the site is setup for robots.