G'Day again Maid,
I'm not sure you can do anything to fix the twitching and loss of voice sync. It could be the source files you are playing that are the problem.
I'm in Australia as well, and running my TV at 60Hz. I have "Display Setting Automatic Change Mode" set Off, so everything plays at 60Hz. I have just finished watching the series Grimm from mkv files with a resolution of 720x404 @ 23.98 fps, and bitrates typically less than 1000 bps. So not the best quality.
I saw some glitches, some twitchiness, and voice sync problems through the whole series. Particularly voice sync drifts during playback. Sometimes stopping the video and restarting it improved things. Sometimes I had to adjust the voice sync as per my above post.
I also see the problem on my Client PC as well. It is also running at 60Hz and a high resolution. I haven't tried playing them using other software (VLC?) to see if they are any better. That may be worth a try, for comparison.
So what sort of files are you playing from, and what sort of quality are they? If you have some good quality 23.98 fps files, do they also show the problem?
Bottom line: If the source files are free and low quality, possibly even including glitches in the files themselves, and suffering sync issues, then MC probably can't be tweaked to make them work as well as original high-quality files.