tivo hd xl: hdmi to tv / optical to reciever question

[agresbach]

My home theater consists of a Tivo HD XL,  Samsung LED tv and a Yamaha 5.1 reciever.  The way I have it setup is, I have HDMI going from the Tivo to the TV and Digital Optical going to my reciever (my wont carry the 5.1 audio coming in from the hdmi through the optical out so whats why i'm going directly to the reciever)
it works MOSTLY except the audio tends to be slightly off (worse on some channels/shows than others).  Has anyone ever experienced this? i'm thinking either the tv or tivo (or maybe both) needs to be set up to tell the tivo to use ONLY the optical for audio and the tv to use the hdmi for only video (i had to setup my ps3 to tell it what to use for audio).
does this make sense? does my tivo have this option? i'm not in front of it or i'd check myself
thanks!