Every 10 - 20 seconds there is a stutter or frame drop for a second or two when playing 1080p MKV files on my htpc. It happens in all programs including xbmc, vlc, power dvd, etc. All movies differ in the amount they stutter. Some stutter more than others, while some don't stutter much at all.
16 gb ram
Let me know if you need any other info to diagnose the problem.