EP2C602-4L/D16 memory error

I bought asrock server motherboard. Two cpus and using all memory slots. After installing second cpu I always have same error "The following channel memory did not pass CPU memory test". Yes I checked for bent pins, unmounted and mounted CPU cooler like 4 times to be sure (Cooler Master Hyper 212x) no way to make it work. I tested all modules independently and all of them are working correctly. Always fails only when channel G is in use. Here the weird thing, all this happens when tower is in vertical position. I lay it on side (like horizontal tower), all works flawlessly. I opened case with Asrock. If this works correctly being in horizontal position, I can discard CPU and memory modules hardware issue, motherboard? This is so weird.