Ok, I gave up after those 3 hours of testing. Here's whats happening: I bought 2 of this (Kingston 2GB 1333MHz DDR3 Non-ECC CL9 SODIMM, Standard 256M X 64 Non-ECC 1333MHz 204-pin Unbuffered) to upgrade my iMac i3 late mid 2010. Infos: -I have Lion installed -I don't use any kind of external hard drive -I have nothing but the power cord connected After installing, everything was beautiful. Lion running smooth. The system was 100% better with the 8gb. I tried to print a document ( I have a Wireless HP Photosmart D110A if that matters ) and the iMac suddenly frozen and started to make those 3 beeps that indicates RAM problem. I booted and in the same printing process it frozen again, with the beeps. Then, I took off 1 of the modules, and I wasn't able to reproduce the problem. Opened a lot of apps, used a lot of ram, printed the document, everything. I was SURE that I got a bad module, but decided to try with the other one. For my surprise, I couldn't reproduce the freeze ! Then again I tried with the 8gb, and while booting, it frozen and beeped. I Changed the modules positions and the problem remained. Then I took off all the RAM and tried 1 by 1 (only 2gb) all the 4 modules. And couldn't reproduce it. I tested all modules with Rember and the Apple Hardware Test. All ok. So, what is going on? My iMac can't handle 8gb ? I can't think of anything ! HEEEELP ME!!! I have seven days to change my new RAM. Should I take it to the Apple Care? EDIT: Running on 6gb apparently makes it stable, no matter what combination of modules I use. With Prime95 I can easily reproduce the problem using 8gb. With 6gb, everything is fine. Might be my logic board failing to recognize 8gb ?