When the computer freezes, this is either a bad/corrupted video driver, or some hardware malfunction (such as bad memory chips, failing hard drive, bad CPU or fans failing to cool the CPU).
Re-install your video driver. Run diagnostics on all hardware. Visually make sure the fans are running properly, or run a diagnostic to determine the temperature of the CPU.
I will run an hardware diagnostic, but i don't explain to myself why the problem occurs only when i run LR.
My Mac works very fine when i don't use LR. It never crashes or freezes in other conditions.
Lightroom may cause more stress on your hardware than other programs.
I have launched the AHT (Apple Hadware Test) which has detected a RAM failure. Now i have to isolate which memory module is affected by the problem. After that i will try to run LR.
I will know you if the problem is completely solved.