Possibilities:

* Over-watering causing a drought response - loss of turgidity

* Under-watering causing a drought response - loss of turgidity

* Salt accumulation in the soil osmotically preventing water uptake - loss of turgidity

* Collapsed soil causing root rot that is impeding root function/metabolism and water transport to leaves

* Soil getting too dry and hydrophobic - doesn't want to absorb moisture

Al

Sounds most likely like possibilities 3 and 4 could be the possible culprits. Now is it perhaps because the newer leaves are 'weaker' than the older ones that they're affected while the older ones are fine?

And now while I'm typing this, I looked at my Spath and one leaf that was flopped over is now fine...weird.

I would try flushing the soil thoroughly several times. Use a wick to help remove excess water if you think the soil is too water-retentive.

Older leaves ARE more rigid than new leaves. I often let Ficus dry down to the point where newly emerging leaves show the first signs of wilt before I water (helps keep leaves small on bonsai - that's not a general recommendation, though). ;o) Older leaves, even on herbaceous plants, develop SOME lignin in the leaf tissues & lignin is what makes wood stiff, so they are a little more resistant to wilting.

Al