Hi Phil,
When you rename a page mojoPortal will autosuggest a new url based on the page name, so if you change the page name but want to keep the original url you would need to restore the original url after it suggests a new one.
Also if the url does change then mojoPortal will try to help you with SEO by creating a 301 permanent redirect from the old url to the new one. But if you're naming things back and forth it can't really do it because then it would cause an infinite redirect loop where the old url and the once new url are redirecting to each other.
In any case where you have url problems, you should check the url in page settings, check if the url exists in the friendly url manager under Administration > Advanced Tools > Url Manager, and you may need to check under Administration > Advanced Tools > 301 Redirect Manager in case it created any redirects that you really do not want. Checking those 3 places should help solve just about any url problem.
Hope that helps,
Joe