Problem editing .mov files in CS6

Hi Adobe folk,

I've uploaded .mov files from my Canon 7D camera onto Adobe Premiere CS6, but after doing some editing the footage in the playback screen is very jerky/slow. I'm working on a brand new iMac OSX and have a RAM of 4GB, so I have no idea why the playback would be so slow.

It's an urgent project that I'm working on, so any solutions would be much appreciated.

Thanks :-)