My money is on one of these three:

1) Exceptionally dry air (not likely, as they can take pretty dry air before complaining)

2) Fertilizer / mineral buildup (more likely, especially if you're feeding but even if you aren't: if you're not watering until a fair amount of water runs out the bottom of the pot, minerals dissolved in your water can build up enough to harm roots)

3) Overwatering (most likely: I know you said you're careful not to overwater, but it's not unusual for the top few inches of soil to be bone dry while the bottom of the pot is still wet. This is especially common when there's no drainage in the pot, or not enough drainage, or if the plant gets watered and then stands in the drainage water because it's in a plastic-lined decorative basket where the drainage water can't drain away. [ahem] It also seems to happen more easily to plants that have been grown in the same pot for a long time.)

I think it's probably time to move the plant up to the next size pot (no more than 2 inches larger than it's in now), by the way. Peace lilies (Spathiphyllum spp.) can be rootbound without dying, but I have yet to find that any of them do better rootbound, despite what everybody says.

Thanks for the quick response.
1. The air in the house is definately dry this time of year. I try to mist, but I'll admit, I'm not too consistent with that.

2. I'm not fertilizing at all. I do take the pot out of the basket when I water it. I take it to the sink and water it until the water runs through. Maybe I'm not letting enough water run through it?

3. Then I let it set in the sink to drain. Even though the basket is lined, I've learned early on that the water does leak through, and then I've got a mess on the bookshelf where the plant sits. So my reasons might not have been due to the health of the plant, but hey...better lucky than smart sometimes. :-)

I'll guess I'll try potting up, and watering even less. For a plant that I've always seen defined as "easy to grow," she sure is a bit moody. :-)

Oh gosh yes. Peace lilies are easy insofar as not requiring a lot of light or humidity, but they more than make up for that by being weird about watering.

Which reminds me -- how were you deciding when to water, before? Lifting the pot? Feeling the surface of the dirt? Watching the orientation of the leaves? Waiting for the leaves to droop? Some of those are better methods than others.

When I first got it, the leaves drooped a couple of times and I went into panic mode and probably ended up overwatering. Then I realized what I was doing, and started sticking my finger in the soil about 2-3 and if it felt dry, I water using the method described above.

Waiting for the subtle change in the angle of the leaves is a better way to tell, with these guys. If you wait until they're obviously drooping, then you've gone too long, but they get a little limp and floppy a couple days before that. It takes practice to see, but it's much better for timing when to water. Just FYI.

It's been awhile, hows your PL doing? I agree w/ Mr. subjective, the leaves are a far better judge of watering than the finger method. And, there is a fine line when it comes to this, for example, I noticed mine was slighly droopy one night as I was getting ready for bed, so I was going to water it the next day, well come morning it was completly wilted, leaves falling over and about 1/2 the height it normally is. I took it to the tub and gave it a good soaking (3 3 gal buckets of water) in less than 1/2 hour it was perking right up. So the key is when the leaves START to droop, not wilt. They don't like to be dry, they really don't like to be wet and they really really don't like to have their feet wet, so drainage is very important. I would take it out of the basket, especially if it's lined. The lining causes moisture and constant moisture isn't a good thing for PL's.