Perpetual Spinning Wheel on Macbook Pro

I need help troubleshooting my mid 2010 Macbook Pro, 15" --perpetual spinning wheel. I created another user account and it worked ok for that account for 3-4 weeks, then same thing happens. I created a third user account and same thing. Wheel spinning so slow that I can nearly access previously created accounts, if at all. Sounds like a software issue, else i would not be able to create other user accounts, right??

Temporary fixes that helped for a while have included emptying cache, deleting cookies but even these quit working after a while.

Running OS X Yosemite Ver 10.10.5

Processor 2.66 GHz Intel Core i7

Memory 8 GB 1067 MHz DDR3

Graphics Intel HD Graphics 288 MB

Where do I start looking for problems? It occurs so gradually that I can't pinpoint out what I am doing to cause the spinning wheel to slow to a crawl and then spin forever. Tasks I do a lot of include: surf the internet, Facebook, email, use Microsoft Office (mainly Word and Excel).

Prior to this, I started getting sudden restarts with the gray screen message: "Your computer restarted because of a problem. Press any key or wait a few seconds to continue starting up." This problem also continues to happen, but the spinning wheel, which started after, is rendering it unusable.

Yes I know the computer old, and it's probably getting to be that time. But I believe it to still be a solid machine ...and I'm not ready.

Any suggestions, anyone?