500 Error messages (DNS)

I have recently become an administrator of a hosting server. I understand the principles behind the server and the DNS, however, still don't understand exactly some of the functions going on.

I am having problems on that I recieve 500 errors (Page cannont be displayed) on some of the sites hosted on the server. I have tracked it down to being a DNS problem (I think!!)

When you try to visit some of the sites on the server for the first time, it will display an error. However, on the second or third attempt on the site will work correctly. The strange thing is that this only happens on some of the sites and not others?!

For example www.rdesignt.com is a particular site. Some of you may recieve an error while others of you wont. Its like that the client has a problem connecting to the dns and once it has all is fine. www.gwsupplies.co.uk for example causes the same problems, however, once this address is resolved all the other sites on the server will work first time.

I really can't work it out?! Can anyone help? Oh and btw... HELLO, I'm new here!

Additional information
Windows Server 2000 SP4
IIS 5.1

I can post DNS setup for rdesignt.com if needed, although I dont think that this is the problem.

Just wondering if someone could list any know issues that can cause 500 errors on iis/dns for the above setup..

Would be v.apperciated as want to work through every possibility to resolve

First, if you are using an IE web browser, uncheck the "show friendly http errors" setting in the browser settings. You should receive a more detailed error message.

There are many, many things that can cause the http 500 error. Bad asp code or incorrect permission settings to access db's is a common problem. Intermittent errors can be caused by misconfigured or overloaded servers.

Im using firefox, however, I am pretty sure that it is a dns problem, as once the client has found the dns server once (although it may take a time, while recieving 500 errors) it works fine. And the errors arn't specific to one site.

Thanks for your responce anyway.