Well, were the 29.97 clips MEANT to be 23.98? For example, were they 24P or 24PA clips? If not, then the conform is doing something to the clips that wasn't meant to be done, so yeah, audio will drift. Unless you convert the 29.97 clips with a high end converter, like a Terranex box (dubbing 29.97 to 23.98 at a post facility) you won't get good results.
PLANNING....you MUST plan for this stuff when you shoot. Don't mix frame rates...especially when trying to MULTICAM. If you plan on multicamming, then don't shoot different frame rates. Making 29.97 into 23.98 isn't a simple thing...as you noticed.
www.shanerosseditor.com
Listen to THE EDIT BAY Podcast on iTunes
[
itunes.apple.com]