The HeadQuarters VFX

Hello all, I have something very strang to which I can't no answer. Prelude: I had a small case workstation running on an I7 4790K, 2x8gb ram, Asrock itx mobo. All things work fine until the aircooler died. So i decided to replace it with a coolermaster watercooler. The situation: From the moment I installed the watercooler, I had an unstable system which only worked with 8gb ram in lane B of the memoryslots. So I assumed I had somehow damaged lane A whilst installing the watercooler. That was five months ago, and until last thurday, my system ran fine, al be it with just 8gb of ram in stead of 16. My SSD drive died on me, so I thought: time to replace the disk and since I'm operating on the machine, replace the mobo with a new one. So I did just that. New Asus H81i-PLUS mobo installed, cpu, watercooler, and this time back to 16gb of ram..... only to discover the system was unstable again and windows setup resulted in an immediate BSOD. I came to the conclussion that this time too only memory slot lane B worked! So with only 8gb of ram in lane B, the system was up & running.... Everything has been tested seperately, and every part is in perfect working order. PSU 750W is working fine, cpu okay, tested with different ram sticks, all is well.... except not in that system. Why? What does lane A in combination with a coolermaster water cooler not to function? And why do both lanes work and give me 16gb when I use a cheap small air cpu cooler? I'm at a total loss on this one. I have repaired and built numerous systems but this one is far beyond my reach it seems. So far no one could give a solution, except: stick to 8gb in lane B, buy a small cheap cooler that overheats the i7 in time or toss the system in the garbage bin. Thank you in advance. Curt