This restarting means that the previous startup did not succeed. In such a case, Plone/Zope is restarted by the so called daemon manager (some number of times). This means, you have a so called "startup problem".
To analyse startup problems, start Plone/Zope in the so called "foreground" mode. You do this by using the command fg (instead of [re]start). In this mode, logging messages are sent to the console/terminal from which the process is started. They contains important hints for an understanding of the problem.