Need help determining if I have bad ram, a bad slot, or something else.

[MaSonZ]

Probably you would have to adjust voltages on the memory and the SPP which is what I just said. In all likelihood it would not be stable at the same settings that you have now. Probably you dont have any need of another 8GB anyway.

i dont have a need..yet.

[MrRiplEy[H]]

Some ram chips that are designed for overclocking can require 2 volts even on regular use and 2,5 when oc:d hard. But that should pop up on memtest immediately.

It's possible that your memory controller is bad or the motherboard has some bug causing instability with all slots occupied. I have a laptop that will not run stable for over 5 minutes if I have both memory slots filled. With just one slot (either one) it runs like a charm.

[Chalenge]

It is possible you have a defective memory controller. Much more likely is the fact that the SB/SPP is in need of additional voltage in order to be stable with four slots populated. Usually (and this is why I recommended visiting the manufacturers support forum) the process of determining stable settings requires the steps I have already mentioned as well as adjusting the timings (loosen) especially if you are going to overclock the RAM. So you should first seek out overclockers with your same MB and memory and read up on how your specific equipment usually responds to having extra slots populated. How my MB or someone elses laptop acts is not going to be a good indicator of what you will encounter. The process is similar but the specifics will be different.