Transcoding AVCHD To Something That Isn't Choppy In Adobe Premiere Pro CS6
What you can do is create an "offline" version of your footage, and then once the edit is complete do an "online".

Pick a codec that is good to edit in, e.g all I-frame mpeg, or proRes / DNxHD or even HDV. The quality isn't actually too important, because before you make your master you delete the transcoded copies, and re-link to your camera footage. Then you can master off the original footage with zero quality loss.

If that doesn't work for you, then the first three codecs I mentioned will be an acceptable compromise. ProRes and DNxHD have the advantage of greater colour depth, at the cost of greater storage size, though since AVCHD is only 8-bit to start off with the advantage might be negligible. Whatever you choose, you want something that is all I-frame for editing.