Hmm...
Ok, you say you can use 2 sticks just fine. Well, divide your RAM into 2 different pairs and make sure you can use both pairs in both the black RAM slots on your board and the blue RAM slots, one pair at a time of course. This way you can make it's not your RAM slots on your board that are causing the problem or that it's just a specific stick of RAM. I haven't seen you state that you've tested both pairs or all 4 sticks individually yet.
I am assuming that when you test your DIMM slots on your motherboard, you want to test a pair of sticks in dual channel configuration which I have assumed was either the two blue slots, or the two black slots. The user guide for this particular motherboard, ASUS M4A88T-M/USB3, is somewhat ambiguous in stating which DIMM slots are paired together for dual channel configuration, what I could get out of it was as I stated above, the slots are color coded. This is the essence of my question, whether the color coding is correct for this motherboard, because if it is, then the slots are not arranged as is typical nowadays where slot 1 is paired with slot 3, and then slot 2 with slot 4, (1+3) or (2+4). On this particular model, if color coding is correct, then slot 1 is paired with slot 2, and slot 3 with slot 4, (1+2) or (3+4). This is my understanding. Obviously, this post is quite some time ago, and has likely long been resolved, or it is no longer a concern for Big L, but I wanted to also point one item out regarding this unusual configuration of DIMM slots. You instructed him to test his RAM a pair at a time in each of the dual channel slots (blue, and black). However, based on his reply, he tested each paired in the wrong slots, as a single channel:
Tested pair 1 in A1 and A2 channels: pass
Tested pair 2 in A1 and A2 channels: pass
Tested pair 1 in B1 and B2 channels: pass
Tested pair 2 in B1 and B2 channels: pass
I was hoping you might see this, mainly so you confirm what I am saying as true, and perhaps if there was a small chance that would be any help to you or Big L somehow in the future, then I wanted to bring it to your awareness . . . for I didn't notice anything in the thread where anybody noting that Big L utilized the wrong DIMM slots.
Here is an image of the motherboard for clarification that includes each slot being labeled (A1, A2, B1, B2):
View: https://imgur.com/gallery/GNQ2OnG
If you are still around these parts and happen to have the time and willingness to confirm my beliefs, then I would be much grateful for your confirmation. However, if you are incredibly busy, then my issue is not of any vital importance, and I would understand if you felt you had no interest to tend to such a request from a complete stranger.
Take care!