Its possible that the dirt needs to be re-placed if you havent re-potted it since you got it. It may be root bound also. Thats my first thoughts, How easy does it come out of the pot when you try to lift it out? Others may have more thoughts on this for you. Good Luck she's a beauty.
Christine

Brown tips indicate salt build up!
You need to leach it really good!

Place it under a tap and run water thru it until it's thoruoghly drenched, then every few hours, flood it!
6 times minimum!
That should flush most of the salts out, peace lilies are really sensitive to it!

I'm not sure where you live, if winters coming I'd wait to to do any repotting, but that will likely help too, if your in Florida or California, leach it good first and then repot.
Likely the leach will be enough, make sure to fertilize it, 1/4 strength, twice a month.
Leach it a few times a year, before repotting, not afte, it washes the nutrients out.
Good luck!