since you said you have other installations running okay,  is this installation the same version as the others? I am assuming that are AbanteCart installations.
what have you done differently with this one?  Any third party extensions?
And code changes you have made that is different then the others?
Since you are rather adamant that it is software,  and those of use who have dozens and dozens of installations think it is more likely there a different issue or something that has been introduced into this installation and not the others. Perhaps there was a glitch when you installed this?  When did you introduce the IP address into your URL? 

Does this happen with all browsers?  Does this happen on a PC  that hasn't been to your site previously?
Have you compared config and rewrites on the problem site to the others?

I have other software running, wordpress, joomla etc. This is happening on all browsers and computers. Client report it to me when it happens and I can confirm its down.

I have turned off cache and now it hasn't happened in 24 hrs. I have been asking where this is stored or how it could be causing it. Any ideas would be great.

a grep of the database and files show (0 localhost or 0 ip entry's in either) so I know it's not coded or hardcoded somewhere.

You using  CDN?

No, and it just happened again. Something with the config is stuck. I know 100% its not the server.

Check setting for your domain. Possibly you have IP address in there.

Nope not that, these seems like some kinda glitch in the software somewhere.