Sunil,
It's "Domain Name System", and basically converts a computer-ese name into a friendly name like "pondboss.com". If there are any glitches in the server software, it can get really freaky. It's similar to phoning home and getting a "this phone number doesn't exist, please try again", when you know your phone is working.
Sometimes software can cause conflicts on a shared server. For example, when a certain script is running, it may be monopolizing or temporarily killing another range of addresses.
It's all way over my head, I just know the basics. I have seen DNS problems on other web sites cause the same symptoms.
