I think something is broken!

My first computer, Dell-W-XP, ran 12 years with the common Dell - W-XP problems…BUT ...everyone said how superior the Apple products were, so I finally, instead of replacing my tired old Dell, bought a 21.5" iMAC (late 2012). ERROR?
Takes forever to load some pages, refuses to open many pages, pages jump around or disappear, photos will drag/drop sometimes and then they won't, passwords last for a day before they're refused, every image I view or copy goes to 'all my files' at least twice, sometimes as many as five times, there are dozens of music items on ' iTunes' plus dozens more of mine that are empty, my wifi printer has stopped taking instructions, my external hard drive has stopped taking back-ups, let's see is that everything…probably not. I had Apple reinstall the OS and it didn't help. I've talked to lots of Apple's really sharp techs, they quickly solve individual problems but can't resolve all the above.
I've had the Mountain Lion for 6 months. Should I box it up and ask for a new one? As I said the Dell-W-XP was a lot of trouble but at least while it was running it worked.

Priority: Seems Really Bad
Name: Ron Grauer
Email: graurog@sbcglobal.net
Problem URL: yahoo.com
OS: Mac OS X 10.8.5
Browser: Firefox, IE &
User Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.8; rv:25.0) Gecko/20100101 Firefox/25.0