Rendering error with wmv

Hi, I'm trying to render a 10 minute Avi video into a vmw file, which takes about 2½ hours but after a while Vegas just crashes with the usual "an error occured during rendering" screen. It seems to be somewhat random, sometimes the rendering goes through fine and sometimes it does not, I've tried closing all other applications and I've also tried setting dynamic ram to 0 and rendering threads to 1 but it does not seem to differ. nothing wrong with my system..

i5 2500k 3,6 Ghz
8gb 1337 mhz ram
HD 6970 2gb

Any form of answer would be appreciated,, thanks..