free : check if your ram is stable using http://www.memtest86.com/

free : check if your cpu is stable using http://www.pazera-software.com/products/mp4-converter/ with threads on auto, if your cpu has`nt enough volts, your PC will freeze shortly after you start encoding

or use aida64 for cache and memory stability test, it will freeze the same as mp4 encoder then it reaches the L caches http://www.aida64.com/