I have been running AC/Decal on my Vista64 for two years (since shortly after Vista came out). XP mode should not be the case (at least by itself).
Question .. Does AC not loose it's connection if you shutoff Decal completely? That would rule out Decal... If AC runs fine without Decal, then start disabling the things you have installed for Decal and see if one of them are being troublesome.
See if it can be narrowed down to what's causing it.