Problems when my mac start
That is a indicator that the Yosemite has a problem with the "boot" sector on your disk. It could be your disk needs repair.

You can confirm that opening your Console (in the utility folder) and type "Disk" in the filter window. If you now see many Disk I/O than that is the problem.

You can try using the disk repair in the Disk utility in your Utility folder.

If that did not help, you should reinstall Yosemite using (holding) the cmd+r after restart.

If the problem is still there, you need a new disk.