I bought a refurbished Dell Optiplex 390 to run a small Minecraft server with and it came with 1 4GB stick of ram.

Today I bought another 4GB stick of ram to use in the 2nd slot. The computer runs fine with either stick of ram in one slot, but becomes very unstable when using both.

Both sticks of ram passed memtest86 in both slots.

Any ideas as to why the computer becomes unstable (crashes, constantly restarts and fails to boot) when both sticks are installed?

 the larger blue stick is what came with the computer and the smaller green one was recently purchased.