I have a thread about this somewhere if you want to search.
Basically it's a bug with REVIT 2009 or sooner (apparently fixed in 2010). The ONLY solution I found, was if I circuited when I was alone in the model and saved, everything holds. It seems when multiple people are sending info to the central file, it's uncircuiting, changing circuits or even balancing the circuits.
True fix = install 2010.
HTH,
~BMC