Strange grid issues. Help?

[Brooklynite]

I've noticed a problem where I have a selection of audio (loop) that is an exact increment of time (say 2 bars). I select the piece of audio, hit command c and then duplicate it over and over( command D). This is a method I often use to copy loops. I've noticed that it doesn't always copy the increment to the grid no matter what I try. I'll even consolidate the clip so that I'm sure it's an exact increment. When I zoom in to the sample level, I notice that, not only it is not copying exactly to grid, but that the length of the clip seems to change slightly on occaision. Is this the clip changing or the grid changing? This sucks because over the course of a minute or two, there is a significant deviation from grid. There also seems to be issues at times when I spot a selection to a bar/beat or time code value. I then zoom in only to realize that it is not exactly on the grid. Anyone seen this before?

Thanks everyone.