Why is DNS returning two different results at random?

I changed my nameserver and host company for my domain 30 hours ago. Now, DNS propagation checks indicate that the correct nameserver is recognized worldwide.

However, browsers on my own machines produce the old site. I tried multiple browsers and multiple devices (Ubuntu and Android), including some that never accessed the site, to make sure that the problem is not caused by DNS caching in the browser or in the machines. Using Hola or Tor as proxy from other countries, I correctly get the new site.

More strangely, some of the browsers occasionally shift between producing the one site or the other.

I suspect my ISP's DNS is giving crazy results, but how could I diagnose that?

Also, strangely, monitor.us is showing the site as going down, then up, several times a day, when as far as I can tell that is not happening. (It is a basic Wordpress site with, for now, no traffic.) That would suggest that monitor.us is also getting strange DNS values. How can I diagnose this?

The output of dig any joshuafox.com shows that the TTL for your domain is 604800 seconds or one week. That is an unusually high value and you might want to change it. Expect the propagation of your new configuration to be fully propagated by the end of the week.

Joshua,

you may even keep a long time TTL as you did if you want to spare your server from too much refreshing - i take it that you had your reasons to set it that high (i would recommend two days - 3 at a MAXIMUM), but when you're planning big changes like that, always reset it back to a few hours (or minutes!) a week before the server's migration.

It may put a higher load on your server, but trust me, it will keep you away from a large range of problems. Let's say everything is running smooth and quiet after the move. Some time after, you see that one of the machines is not responding the way it should. It may be a hardware bug, new network factor (did you test your application's behavior under the new load-balancers?), a power-related glitch, it could be a LOT of things. In all those scenarios, your ability to make things transparent to the users as quickly as possible by reverting to the old infrastructure will be based in your DNS TTL.