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Macs are notoriously 'picky' about RAM. Just because modules are advertised as meeting the specs doesn't mean they will work reliably.
I learned the hard way with a RAM upgrade from a manufacturer who did not guarantee Mac compatibility. They did replace them once, yet the RAM still didn't work without frequent Kernel Panics.
RAM from Apple, Crucial, and OtherWorldComputing (www.macsales.com) seems to work well for most people who discuss it here in the forums. With other brands, sometimes it works, sometimes it does not.