I bought a refurbished 3.06ghz Intel Core i3 iMac, with 4 gigs of ram, has the 256mb ATI graphics card. Running Yosemite 10.10.5 The computer was supposed to come with 8 gigs of ram but was sent with only 4 so I contacted the seller and they sent two Hynix 4 gig sticks. Thanks! - but one 4 gig stick was bad. Was not recognized in the refurb or in my other iMac that is the same model. The other ram the seller sent seemed to be fine, but the last two weeks I'm getting the loud '3 beep' alarm on occasion, that usually signals bad ram. (I had also installed 2 2-gig sticks from the other (same model) iMac to put the refurb iMac up to 12 gigs of ram, and those had been in the other iMac for over a year without a problem.) I ran Tech Tool Pro ram test last night and all ram tests as OK as well as the graphics card. Could this be a logic board problem? I guess I should take out the one 4 gig stick and see if it continues to happen. It's kinda strange though, as it will happen once - I'll reboot, zap the pram and then the iMac is good for a couple days, running Photoshop, Word, online etc. - then it happens again.