Newbie DNS help!

Sorry about this, I’m getting a bit white over this... can you help?
I am currently migrating sites to hostgator. I made the DNS changes yesterday. Since then I’ve been checking the sites

now here is the crazy bit... when checking these links the sites are sometimes down and sometimes up!
Also if I type one of these domains in full form, www and the site is down, then take away www to leave domain the site re appears and vice versa!
Is this “flickering” normal or do I have a problem?
I would appreciate any help as i think I’m going mad!
Cheers,

Gil

It's not normal. Usually it is a caching problem if you're changing hosts, or the records aren't set up properly. It would be guess work without a domain to actually look at.

Thanks all, problem now sorted! Incorrect nameserver configuration!
Gil