Have you had a look at FCLK speeds and SOC voltage, these 8000 series CPU are supposed to be able to run upto 2500FCLK, but for 6000mhz ram you should only need to set it to 2000FCLK, set soc volatge to 1.25v which will be perfectly safe, however you might be limited by the board, not sure how much you can change on a 620 series board, and how much is locked down, ive never used one.
AMD state the best config is 3:3:2, 3000mhz/6000MT/s RAM, 3000mhz MCLK, 2000mhz FCLK.