I've been working in CS5, with an older FLA I originally built in CS3. On the timeline, I have different MCs and button symbols which need to stay in the same coordinate position. Usually if I notice some sort of 'wiggle', I will go and check the position coords by hand, updating the x/y position by typing numbers directly in the Properties panel.
However today, I repeatedly saw the coordinates change -by themselves- after I typed and hit Enter. For example, typing '165' in the Y field changed immediately to '164.95'. With one symbol, this behavior continued, no matter what I typed, with the number changing by 10 pixels sometimes, every time I just selected the symbol. Not only was I not able to keep the symbols at the same coords (I tried making them all 164.95), the 'automatic decimal' numbers would actually change further, as I was trying to correct them. I was only able to resolve this by creating an entirely new MC symbol in the Library, and placing it on a new timeline layer.
I've noticed this sort of behavior before in CS3 and CS4 (the CS4 Alpha slider never wanting to go to 0 or 100%), but never to this extent, or as consistently. It's gone from an annoyance to a genuine problem in my workflow.
My guess is that you are encountering the matrix, which has always been an issue in Flash but is encountered/revealed more often with hot text with the decimal points showing the actual value when it didn't all the time to this degree in earlier versions (or adjusts their values much more often by dragging - doing more rounding, putting the actual value further off). You can see more information about this in my comment on this post here: