I bought a 2011 iMac on ebay--2.5ghz processor with 8GB RAM, 500 GB hard drive. It was listed in excellent condition and from a high rated seller. I received it today and was glad to get it. However, it runs so slow... programs won't open, close, they freeze up then say not responding. I erased the hard drive and reinstalled the OS which is El Capitan. It goes through these slow spells where nothing will run right and then for awhile it will run ok with decent speed. But now trying to go into the App store, its locked up again with the circle twirling...I have had other macs and they have never been like this. Anyone have any advice?