It's a mac problem, but I'm posting it here because I suspect the problem can be solved by a change in gpu.
I just did a clean install of Mountain Lion in an early 2009 mac pro from Snow Leopard. System specs are:
CPU: 2.66GHz Quad-Core Intel Xeon 3500 series
GPU: NVIDIA GeForce GT 120
In Snow Leopard, HD video would play both in QT and FCP perfectly acceptably, but in Mountain Lion, HD video stutters in both QT and FCP, but plays acceptably from streamclip. I've seen at least one other source indicate that the problem is with the gpu:
But I was hoping to find that some others had resolved this problem by switching gpus to maybe the radeon 5770 or 5870 before I pulled the trigger on the purchase myself.
Resolved this issue first by downgrading to Snow Leopard to dodge it, but then installing a Sapphire 7950 Mac Edition and upgrading to Mtn Lion.
All video-playback issues have disappeared.