I'm actually having this exact same issue with my i7-7700k. I've tried just about everything I can think of to fix this, but it's been happening consistently for about 6 or so months, maybe longer. Everything runs fine for about 12-24 hours of uptime, and then as soon as that threshold is breached, it drops to .76-.80 ghz, and the only way to restore normal clock speeds is to shut down, throw the kill switch, and let it sit for a while. Hoping someone here can help.
I've had people on other forums suggest it's a thermal throttling issue, but I find this quite hard to believe. Please see the attached photo for confirmation. My CPU temps float around 24 degrees Celsius and rarely surpass 45-50 C during heavy loads.
Other specs of possible importance:
CPU: 7th-gen, i7-7700k, no OC
CPU Cooler: EVGA 280 MM CLC running at full speed
GPU: MSI GTX 1080ti
PSU: Corsair HX750i, Platinum Certified
Ram: Corsair 3200mhz, clocked at 2133 or 2600, can't remember.
Mobo: MSI z270 m7 Gaming Enthusiast.
I've reset my BIOS multiple times, updated it, reinstalled windows, checked my cabling, and just about anything else I could think of. Does anyone have any ideas as to why this might happen and only after a certain amount of time. It's currently running in it's normal range: 4.1ish-4.5ish with intel turbo. I'm at a complete loss.
CPU Clock Speed Issue.jpg 1.3 MB