After doing research, I have found that the only stable way to edit AVCHD-files from my Canon C100 is to use a 1080p timeline and interpret the footage, conforming it to "No fields, progressive" rather than leaving it as is. Then I must TURN OFF the Mercury Playback Engine(a real time saver at times), to do my editing in a stable way, without interlacing artefacts.
Can Premiere in the future have better support for C100-media? Or rather, AVCHD 25PF media in general. So you don't have to conform the media after importing, and so that you can use all the benifits of the MPE, without getting artefacts and/or general bugs.
Is there work going on at Adobe for better implementation of this type of media? Or are Adobe only blaming Canon for using a "non-standard" codec? =)