I wonder if it's possible MineCraft is taking up too much of his single core that the OS is running on too (just a thought).
Question to all, perhaps he could try a program to separate the process into its own core?
AFAIK - Minecraft can't take advantage of multiple cores/threads, so maybe having its own core would be beneficial?
I've heard success stories from doing so, but I'm not too knowledgeable about how it works, what to use, what it could affect, etc.