I've recently built two almost identical 1950x builds.
Gigabyte Aorus mobo - F2 BIOS
Corsair 64GB RAM 3000MHz using XMP profile
960 EVO nvme SSD
The first build was consistently running all cores at 3.7GHz under full load, and was operating like that for as long as I had noticed it and had been monitoring it. I could also see that the clock speed would progressively drop when the temps rose to over 62 degrees (tdie). I tended to set the Corsair link profile to 'performance' when I was intending on running it at full load. My friend with the same build also experiences the same behavour.
I have since built a second one, and on the same day I also had air conditioning installed in my office.
We're just coming into summer in Queensland, Australia and I was becoming concerned that they may overheat.
My office's ambient temperature is now set to 23 degrees, which is considerably lower than when my PCs were heating the room up (ambient was probably late 20s) and I've set my corsair link profile back to 'balanced'.
The unusual thing, and what prompted my post is that both computer's CPUs are now only clocking up to 3.5GHz. Not sure what could have caused the performance drop, they're both nice and cool and are both averaging around 55 degrees under load (tdie).
I have two custom water cooling kits waiting at the post office which I intend to install - originally it was to combat the heat, now I'm hoping for silence and/or good cooling when the air conditioning is off.
What are you using to monitor clock speeds? What voltages are they running at?
I find that using CPUz, which is a static display, can be misleading. Try HW Info or HW Monitor. They track high, low and current.
As for why they won't run 3.75 vs 3.5 now.....I'm not really sure. Was it the same program both times? Same load?