I've got the same board, have been messing with it on my test bench for a few weeks now. I've got the 2x16GB G.Skill 6000C30 expo kit.
I have it running 6400C30/2133FCLK with tightened timings and it is rock solid. A little slow to post, but that's the only complaint.
I did notice when you enable EXPO it sets some voltages a little aggressively, which I've toned down based on the values that have been working on my Gigabyte board. Particularly VSOC it set to 1.3V (!) I set to 1.2V for 2000/2066 or 1.25V for 2133. DRAM VPP it set to 1.95V, I set to 1.8V. And CPU VDDIO it set to 1.4V, I set to 1.1V. Lowering these voltages ended up causing the automatic voltages for VDDG and VDDP to lower themselves, as well.
These may or may not have anything to do with your intermittent issues, but it seems like on ryzen platforms, high voltages can cause instability just as low voltages can.