Hi,
I have exactly the same issue when rendering into AVCHD, 1080i. End result gets choppy after about 2~3 minutes.
Using i7 920, Win7-64, Vegas Pro 64, GForce 285, 6Gb. No problem viewing any of the source files.
I tried rendering smaller parts separately then collating them, same poor result.
I will contact Sony to increase the number of voice to be heard.