About OS X Yosemite 10.10.3

i am using OS X Yosemite 10.10.3. This is working well but some time they take more times to startup.One Times it doesn't startup only seeing loading but not start then i go to service center then he say your os is not working i will install this os again after reinstallation then it woking well.So, i say that resolve some error problems in OS.