It is possible that your host has a DNS server issue or some internal routing issue, (or other) for your domains -- ask some friends to try to open them when you have problems and see if they work for them OK ...

If they work for them ...

On Windows open a command prompt and type >nslookup domain_name

On LINUX in terminal type $dig domain_name

See if your DNS resolves to the correct server IP CONSISTANTLY - If it does then it is not a DNS issue

If it resolves to 2 IPs then there is an error somewhere ... start at the registrar then all the DNS servers that that domain has entries for -- good luck!

If it doesn't resolve -- add 8.8.8.8 (Google's DNS Servers) to your DNS resolvers or some other Public DNS Servers
http://www.bing.com/search?q=public+dns+servers