Very interesting!
I have exactly the same problem.
I could not add more than 2 cores before the stuttering began in Windows, but I did not test the 3,5,7 combo, which actually works, so now I have 3 working cores, so thank you for that.
Also, MacOS runs as butter with more cores for me as well.
Any new findings in this matter, as the last post is over a year old?