I have used Camtasia Studio and Recorder for around a year and a half now for both screen recording and editing MP4.
I have never had this issue until today, when I imported a video i'd converted from AVCHD to MP4 (Which I do all the time and it works fine) into Camtasia Studio, but all of a sudden the preview window suddenly went choppy. I've tried restarting the program and I have checked the video clip to ensure it converted correctly etc.
Intel I7-3770 @3.4 GHZ
16GB of RAM
64-bit Windows 8.1
Nvidia Geforce 2GB GTX620/30 (I cannot remember the exact model number)
The video was as I mentioned originally in AVCHD. I converted it to MP4 using a little program called 'AVCHD to MP4 Converter' and it has always worked fine previously. The video is in 1080p recorded at 30fps.