What is the cause of 500/503 error in Apache/Mono?

I have web services deployed in Apache/Mono server in my production. It is working fine, but sometimes it throws 500 error (internal server error) in my Apache log in a random manner (i.e. it throws this error irrespective of web service) and the consequent service calls after that throws 503(Service t开发者_如何学Pythonemp unavailable) error, which makes my Apache/Mono crash. I need to restart my Apache/mono to run the service.

So my questions are:

1. What is the cause of the 500 error (internal server error)?
2. What is the cause of the 503 error (server temp unavailable)?
3. Is this errors caused due to Memory overflow in server?
4. Is this problem due to Apache configuration problem or code problem?