Macbook Pro Client Instability

Hi! Looking for some problems ive been having with gameplay and my client across multiple patches now. This problem comes and goes, and I dont know how to replicate it so any thoughts would be helpful. Basically more often then not, I will get through champ select just fine and enter the rift, but not long after, my frame rate begins to get super wonky. It will oscillate every 1-2 seconds between 60-100 fps and 0 fps making it utterly unplayable. ive done everything mentioned on the Frame rate issues page, and attempted to run the Hextech tool, but running this never finishes no matter how long i let it run (ive literally let it go for 48 hours). So i dont really know what to do. Im currently running Mojave and have no other issues. Thanks!