can not create domains

[kpsinghl1]

I created some domains using the default way.

Some of them are up and some are not a default plesk page appears saying that hosting for this domain is not configured on server.

I deleted a domain and used another IP on server for creation but an error message appeared saying that DNs records already exist.

Also one of my domain appears and the goes down periodically after six hours or so.

What I am doing wrong and how should I rectify it.

Thanks.

K P Singh