Since updated to Yosemite, always problems.

As I said, the computer stuck at grey start page with an apple logo there, after performing a safe boot, the computer survives but all the stuff will be showed slowly and you can easily see lines scanning from the bottom to the top.

And then the next time, it happens again.

More often it took a long time for the computer to react, even shutdown and start...

My computer was bought 10.2011, and so far it looks like it doesn't enjoy Yosemite....

Any suggestions to make it better?

Thanks.