HTTPS and error HTTP 503

HTTPS and error HTTP 503

  
Hi guys,

We're trying to implement HTTPS in our web app. For development purposes, we've generated our own certificate.

Everything is running smoothly except that, once in a while, after a few minutes, we start to receive http 503 error. I don't know why!! 

Our server is IIS.

Any help?
In your IIS logs, what's the error code substatus of the 503 errors you're seeing ?
I've found out that the trouble is that application pool is stopping. Now I'm trying to figure it out the reason why that happens