Are you using MSI Afterburner or anything that monitors system/GPU power.
Its a bit of a random issue, but POWER monitoring specifically has been a cause of stuttering in some platforms for quite some time (with reports going back to the middle of last decade, was mentioned in some Nvidia driver update reports too at the time).
Maybe try uninstalling ALL your monitoring software that is running in the background, just have MSI Afterburner, but explicitly got into the settings and turn off power monitoring on the power tab and reboot; incase you're system is just being polled either too aggressively, or to a part that is not taking it well.
As above, you're kinda trying to work through and eliminate things as you can now; given the CPU has been replaced.