Is it possible that a stall coverter will loosen up as you break it in?

 

The only way they loosen up with time is one you add power, two the fluid shears down to a thinner grade, neither of these actually loosen it but it will feel looser, or three the fins begin to laydown internally and it will get looser and ineffient on it's way to selfdestruction and taking your tranny with it.
This is another benifit to buy converters like Edge, Yank or Vig to name a few if the stall is wrong for the car they will restall it for you sometimes even free inside a certain time period from purchase.