Debian VPS: websites stop working sometimes, fix that

I have a VPS running Debian. Lately, a few times a day, the websites running on that VPS stop working. It becomes 'unavailable' in the browser. An example of a website running on it is: [login to view URL] Currently it's not available. If I reset the VPS then the sites will start working again. SSH access is still available when sites are not. Selected freelancer will get root ssh access to find the problem and fix it. Websites will need to run without problems for 3 days in a row to confirm the problem is fixed, after that job will be accepted as complete. Thanks.