Why does my Intel HTPC not work as well as my AMD?

I recently built a second HTPC and opted for Intel based on the high praise of the Sandy Bridge chips.
I was a little apprehensive at first as I had heard about Intel not working properly with 24p but based on several threads defending Intel and the good word of some respected forum members I understood it would not be an issue in XBMC.

See my specs below.
My AMD machine is pretty responsive navigating Aeon NOX.
-With render set to Auto Detect and hardware acceleration on the 1080p stuff is super smooth including the infamous "Birds" sample.
Hence with AMD I stick to AutoDetect and use Hardware Acceleration without issue.

My Intel machine is a little more snappy than the AMD at navigating Aeon NOX.
-With render set to Auto Detect and hardware acceleration on the1080p stuff is also super smooth including the "Birds" sample. However, there are a couple of movies which show severe macroblocking (not sure why those specific movies) so that is a dealbreaker.
-With render set to Software and hardware acceleration turned off there is also stuttering and dropped frames on high bitrate material (about 70 dropped frames in the Birds sample).

Can anyone help me out? With many people defending Intel I'm sure there must be a way to get everything running as nicely as the AMD.

Thanks

HTPC#1
Athlon II X2 240e 2.8GHz
Gigabyte 880GMA-UD2H (Radeon HD 4250)
4Gb GSkill DDR3
Running OpenELEC (XBMC Eden)

HTPC#2
Intel Pentium G860 3.0GHz
ASRock B75M-ITX
4Gb G.SKILL DDR3
Running Windows 7 x64 (XBMC Eden)
[all latest drivers are installed]