i'm having very similar (if not identical) issues and have been trying to get to the bottom of it for a few weeks.
i'm not overclocking at all. my build is...
MOBO > ASUS Sabertooth 990FX R2.0 (BIOS: AMI V.: 2501 - 04/08/2014)
CPU & Cooler > AMD FX-9590 | CoolMaster Seidon 120M
MEMORY > G.SKILL Ares Series 32GB (4 x 8GB) DDR3 1866
POWER SUPPLY > Antec High Current Gamer 900W
VIDEO CARD > XFX Double Dissipation R9-290 4GB
HDDs/Optical drives > System: Silicone Power S60 240GB, Optical: LG Black 14X BD-R
SOUND DEVICE > On-board
O/S > Win 7 Ultimate
Case > Antec Three Hundred Two
Fans > back: in - 120mm radiator * top: in - 140mm* right: in - 120mm cpu * left: out - 120mm gpu
always freezes about 5 -15 mins after boot. no temp anywhere gets above 50C
i'm troubled by the fact asus says this is a 140W MB...