If you were having a heat problem that would show up right away. I would start with the memory first. Try one stick and than try the other stick.
Or you could run memtest on the memory. If the memory is OK. I would start looking at the video card. Plus all your parts are Compatible with ea other.
Gigabyte-Ga-x99-UD4 i7-5820K 3.30Ghz G.Skill 16gb DDR4 2133Quad, His R9 290X IceQ X2 4GB, IN Win Glacier 900w Cooler Master HAF 932, 2-240Gb SSD Array Win7 Ult-64bit 39" LED TV
GIGABYTE-GA-990FXA-UD5R, FX-9370 4.4GHz AM3+ 8-CORE, Mushkin 8gb DDR3 1600, EVGA GTX 480-OC 1.5GB, Ultra 750W PSU, cooler Master HAF 922, Win7 Pro 64bit & Win-10 64bit - Play around with computer