If the page is down on the first check and you don't do anything, how do you expect it to come back up within 5 minutes.
If it were me, I simplify it by checking every hour and immediatelysending an email if it's down.
I would say in all honesty, don't bother doing this - certainly not with ColdFusion. Two reasons why, firstly because there is already software out there to monitor sites far more thoroughly than this that you can get for free, and secondly in my experience PHP (which you're testing) is generally more stable than ColdFusion (which you're testing with). If anything in your IIS / Java / ColdFusion stack goes wrong, you have no monitoring at all, and you don't know it. There's a reason monitoring software is normally quite simple behind the scenes, and is a standalone service or executable with very few dependencies.
I'd say you're just as likely to lose the CF service as you are the PHP website. And as Dan quite rightly says, why are you monitoring a site where it doesn't matter if it's down within five minutes and is capable of fixing itself? No site should ever be down for five minutes regularly these days.