Encountering problems after updating to Yosemite

Question

Hi! I updated my iMac (Refurb 2011 27" iMac, i5, 2TB HD, 8GB RAM, 1GB VRAM), Yosemite from Mountain Lion, after getting a new iPhone 5s last week. I had to update iTunes, and to update iTunes, I had to update my OS.
Problems after downloading:
- USB ports are intermittently working.
- Illustrator CC will freeze my entire computer (rainbow beachball not even rotating). Happened twice already. Have never experienced a complete freeze before.
- iMac turned itself off from sleep mode. After arriving home from work, the computer would not turn on. I thought perhaps I was having a power button issue (I've already had the shell replaced once), but after unplugging, waiting 15s, and replugging in, it turned on. I guess it's possible the power went out, but I doubt it.
- Last night, it restarted from sleep mode after 5-10 minutes (freaked me out since I had gone to bed). I have read this is a common problem; surprised it hasn't been patched yet. I just turn it off now.
- Every time something happens and I have to restart, my background is back to default. (minor gripe)
I have done PRAM and SMC resets.
Is it worth going back to Mavericks, or are these addressable problems? I thought since I had waited some time since release, the kinks would have been worked out by now (it's been 6 months, right?). If I was just using this computer for fun I wouldn't mind as much, but it's really messing with my freelancing work flow.
Thanks for any help!

Answer 1

Reinstalling OS X Without Erasing the Drive
Boot to the Recovery HD: Restart the computer and after the chime press and hold down the COMMAND and R keys until the menu screen appears. Alternatively, restart the computer and after the chime press and hold down the OPTION key until the boot manager screen appears. Select the Recovery HD and click on the downward pointing arrow button.
Reinstalling OS X Without Erasing the Drive
Repair the Hard Drive and Permissions: Upon startup select Disk Utility from the main menu. Repair the Hard Drive and Permissions as follows.
When the recovery menu appears select Disk Utility and press the Continue button. After Disk Utility loads select the Macintosh HD entry from the the left side list.  Click on the First Aid tab, then click on the Repair Disk button. If Disk Utility reports any errors that have been fixed, then re-run Repair Disk until no errors are reported. If no errors are reported click on the Repair Permissions button. Wait until the operation completes, then quit Disk Utility and return to the main menu.
Reinstall OS X: Select Reinstall OS X and click on the Continue button.
Note: You will need an active Internet connection. I suggest using Ethernet if possible because it is three times faster than wireless.
Alternatively, see:
Reinstall OS X Without Erasing the Drive
Choose the version you have installed now:
OS X Yosemite- Reinstall OS X
OS X Mavericks- Reinstall OS X
OS X Mountain Lion- Reinstall OS X
OS X Lion- Reinstall Mac OS X
     Note: You will need an active Internet connection. I suggest using Ethernet
                 if possible because it is three times faster than wireless.