If core drops when you raise the mem clock - you run into the powerlimit - increase it.
I have seen similiar things on 1 of my 2060S, all of this cards run at 45mh, on 1 card, if i raise the mem - the coreclocks drops to 750. For me it makes no sense because the card only use around 120w, but the powerlimit in the bios is a little bit lower than all other cards and i can´t increase the pl over 100% it is kept there.
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When the core drops to 750 on that one card, if you're using HiveOS or linux, run "nvidia-info" and it will report if the card is being throttled and the reason for it.  If the card isn't reporting any throttle but you can't manually push it above 750 MHz then it most likely is a power problem and that particular card is sensing decreased amperage which will cause it to limit without reporting. I've seen 1000 watt PSUs that can only supply 700 watts consistently and will run into issues when loaded beyond that -- it can run 1000 watts transiently to provide power for a rapid spike load, but just not for more than a few seconds.