Sluggish playback in FCP

Hi,

I've got a problem with sluggish playback in FCP

My config is as follows:

FCP 7.0.3

Model Name: Mac Pro
Model Identifier: MacPro4,1
Processor Name: Quad-Core Intel Xeon
Processor Speed: 2.26 GHz
Number Of Processors: 2
Total Number Of Cores: 8
L2 Cache (per core): 256 KB
L3 Cache (per processor): 8 MB
Memory: 16 GB

eSATA to 8T LaCie HDD

When I hit space bar, the beach ball appears, and when it's finally playing, the video in the canvas just stops playing, but the sound keeps going. The footage is from Canon HD.

I've tried cleaning the cache and preferences in mac, but the problem persists. Could anyone help me with this problem? Thanks in advance.