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I upgraded the RAM for my old, mid 2009 MacBook Pro.

As I couldn't find 1066 MHz RAM, I bought two 4GB DDR 1333 MHz RAM modules. They are recognized by the MacBook, but it didn't work well and the MBP was slow sometimes. However, when I just replaced 1 RAM module, the machine worked really fine with RAM speeds at 1066 MHz.

Is it possible to adjust the RAM speed?

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> but it didn't work well and the MBP was slow sometimes — might be because all the mid 2009 MacBook Pros only support 1066 MHz… –  slhck Sep 13 '11 at 18:11
    
@slhck this comment should be an answer. –  WrightsCS Sep 13 '11 at 19:03

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You said you have a mid 2009 MacBook Pro. Let's see on Apple's offical site, MacBook Pro: How to remove or install memory. There we go: All mid 2009 MacBook Pros use PC3-8500 DDR3 1066 MHz type RAM.

but it didn't work well and the MBP was slow sometimes

This happens when you use RAM not matching your specifications. It might work, it might not work. There's no real answer. Even if it (presumably) works, experiencing slowness or even a totally unstable system is not unheard of.

When you mix 1333 MHz and 1066 MHz, the MBP will presumably fall back to 1066 and run on this speed instead.

This forum thread suggests that some people got it working. Here's another Q&A from iFixit that suggests it might not work.

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