Ah ok, so not all modules are seen.
Check the CPU for bent pins, as the memory controller is inside the CPU.
Else try some el-cheapo memory, as we seen all to many problems with Gskill.
Mem installed 8GB ( you can use only 3,47GB)
Typical this means bad memory.
Windows XP and Vista used to crash over this, Windows 7 simply avoids the bad memory and runs.
In short, borrow some known to be good memory from a friend and plug that in, see what happens.