Here's some backstory about the computer and where I'm at with it. I know it's long, but I just like to explain things.

Over the past couple weeks I've been messing with a 15" Mid/Late 2007 MacBook Pro (EveryMac link) that has always had some issues. I got the computer from a scrap pile years ago, but physically it's in decent shape. I definitely got my money's worth though - the previous owner had upgraded the computer with a 480GB OWC SSD. The only thing I don't have is the original battery. I don't remember if it was missing to begin with or if it was swollen and I disposed of it, but either way I don't have it.

The computer has always had some SMC issues. I've now gotten it to the point where it runs fine, and it's been pretty reliable. I need to upgrade the RAM though, as 2GB of DDR2 is just not enough for El Capitan. This leads me to my current issue: RAM.

The computer came to me with 2 Corsair Mac Memory modules in it - each stick is 2GB of DDR2 667MHz. Each of these modules works perfectly fine when installed individually in this MacBook, and they both work fine in other computers. I've had one of them in my PowerBook G4 15" for some time now and it's worked fine. However, I can't get the MBP to boot with both modules installed. Both slots are working, and either stick works in either slot, but it will not boot at all with both installed together. The power light flashes repeatedly, though for some reason it isn't beeping like I would expect.

Does anyone have any ideas as to why this machine is being picky about RAM? I'm planning on getting out some other DDR2 modules and trying them, but I can't see why these won't work, especially since they were installed in this computer when I got it.

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Edit/update: I tried a different 2GB stick from another laptop and it's working fine in combination with one of the Corsair modules. Not sure why, but it works, so I guess this machine is just picky about memory.