Illegal AC-3 Stream error

[Grass-hopper]

On ripping a DVD i have the choice of 2 audio streams - a 6channel one and a 2 channel one. Have ripped both of them along with the movie to my HD. Using tmpgenc i then try to mux the .m2v file with the the 6 channel .AC3 to convert it to a low resolution dvd mpeg file, but i keep getting this illegal ac-3 stream error. This same error happens when i try to use the 2 channel audio track as well. If i just try to convert it without muxing them first it gets to about 2 minutes into the film then a windows error message comes up -tmpgenc has encountered an error and must close - the details just go on about floating point errors etc.

Any Ideas??

Cheers;)