MacBook Pro 13" with 8gb ram and an i7 processor..
havent a clue what the graphics card is but it starts grinding and lagging after 3-4 hours besides getting very hot to touch.
Mac is 4 years old.. cannot upgrade hardware just soft ware in this.. frustrating to play at times.
This sounds like an overheating issue to me.
Try to limit foreground FPS to 30 and background FPS to 8.
After I did that my laptop was not overheating anymore in D3. If it will work fine that way, maybe up the FPS limits a bit.