I get exactly the same thing in Vegas pro 12. New computer… no other codecs that I know of installed.
The render option “AVCHD 1920×1080-60i” defaults to 16Mbps. Using the default, rendering short sections of projects works fine. Rendering longer (3+ minutes or so) sections causes video/audio sync issues and stuttering. Not only that, but on one project (3 and a half minutes long, 4 video tracks, one audio), right clicking on the rendered output file and looking at the properties, it showed the length as over 2 HOURS long and the bitrate at 300Kbps or something. Yet, it plays to the halfway mark flawlessly and then screws up.
Just lowering the bit rate to 15,000,000 in the template and then rendering produces a file that plays perfectly!