I would also check more frequently, 1 hour downtime goes below 99.9% a month... the ip...the ns1 and ns2... same thing...
Like ns1 on my server points to a certian ip
ns2 points to a second ip and so on, so you can check either
What software are you using? You should be checking for service uptime and response time. We use Nagios and are very happy with it. you can set up distributed monitoring and all, very cool. Great mailing list community also