I've been struggling a lot lately with keeping a stable CPU frequency under load for my 2.8 gHz 2015 MacBook Pro 15". I've tried numerous methods, including resetting SMC, NVRAM, hardware diagnostics, cleaning out the internals, changing thermal paste, none of which seems to help in the long term. This causes a very stutter-y experience while using the computer. I've noticed that this happens somewhat less frequently when the discrete GPU is not used, but this still happens from time to time.
I've tried booting into BootCamp and checking the CPU status using Intel XTU. It seems like whenever the frequency reaches the top it thermal throttles for a split second, even when temperatures are in check (I'm talking 70 degrees C). I monitored iStat temperature sensor data but it shows nothing outrageous either.