Hi Christine,
When a peace
lily droops it is almost invariably because the soil has become too dry. This is a
plant that is pretty hard to overwater. It is quite possible that yours needs a thorough watering every 2 to 3 days. Try that and I suspect that you will see an improvement.
As an alternative, you can move your
plant into a pot one size larger. However, repotting will deter any
flower production. Peace lilies must be quite potbound to
flower. Think about it: yours stopped flowering after you repotted; then it started to
flower again recently after it had become potbound once again.
My advice is to keep it in the same pot, but water more frequently.
BTW, a north-facing window is fine for peace lilies, as the success of your
plant amply demonstrates. Keep it where it is!