dying after molting

Question
This is our third hermit crab to die after molting.  We have had this particular crab since August 2006.He has always seemed to be our healthiest, most active crab.  None of the others were even able to change their shells successfully - which he has done four times now. I noticed yesterday that he had molted and he looked pretty good - which I was glad about since the others never made it past this stage.  At any rate, this morning I found him on the other side of the tank with most of his body(legs) out of the shell, just limp and non-responsive.  He was so active and social and I thought he'd be the one to make it past this stage successfully.  Why does this keep happening?
(He is in a glass tank (L:16",W:8",H:10"), with about 1.5" of sand on bottom.  He has a water dish (plastic) and another water source (a shell with a water soaked natural sponge).  We feed him FMR Land Hermit Crab Food and often FMR Land Hermit Crab Treat.  The temperature strip on the tank usually reads 78 degrees.  In the winter we had a warming light.  There are currently three extra shells in the tank with him (but he did just change his shell about a month ago).  There is also a small "climbing" area made of legos.  There is also a mineral stone in the tank.  I think it's for calcium(?)  The pet shop owner recommended it at the time.  We also have a mister which we use occasionally when the tank seems too dry.
Thank you for taking the time to respond.  I really would like to be able to see our crabs make it past this stage.  The boys are always so enthralled when the molting occurs and then, so upset that it always seems to end in death for them.  So, whatever you can offer to help us would be appreciated.

Answer
From what you've told me, it seems that the problem may in fact be stress. Hermit crabs need a little bit deeper amount of sand in the tank (2 inches or more) to be able to go completely under the sand, or they need a hidey hut to feel more secure, hermit crabs are fragile, and stress kills them just as easily as a disease would. If you get another crab (you really should get two, multiple crabs helps reduce their over all stress level too, they are herd animals after all) and you suspect it is about to molt, it slows down, burrows more, drinks a lot more water, etc. don't bother it or allow your sons to touch it, molting is an extremely stressful time, because the only protection they have, their hard outer shell, is completely gone, they could be killed extremely easily during this time and they know it. Also, a hermit crab's tank should ideally be warmer than 78, it should be in the middle 80's or above, the tropical regions they come fromt rarely get into the 70's even in the winter time, a good range would be 85-90, but no hotter than 90, because hermit crabs do stay in the shade on VERY hot days, and above 90 is too hot for them. also, you may consider instead of the shell with sponge, get another small water dish and make salt water for your hermit crab. You can make it with marine aquarium salt, freshwater aquarium salt, cooking seasalt (make sure it's SEA SALT) table salt won't do, because iodide is a heavy metal and it kills crabs if they get it in their systems. "pickling" salt is not iodized and it's cheap too, if you can find some near you that the ingredients just say "salt" with nothing else, use that, don't get anything with an anti-caking agent. Mix the water in the bowl until it tastes very very salty, then put it in there, the salt water has vital minerals and electrolytes that hermit crabs can use when molting. Instead of the block, you may consider crushing it into a powder in a ziploc baggy, hermit crabs don't naturally understand scraping off bits of calcium from a block, I don't know why companies make "blocks" for crabs, they can't understand it, it's not natural. Anyway, take the crushed block and sprinkle the powder on the crabs food every other day. For extra calcium, you can put a leaf of kale in the bowl, or find some rabbit hay, alfalfa, crumble it into a fine powder to add to his bowl every other day. Calcium is vital during molting so they can grow a new outer skin. And don't worry about the sponge, hermit crabs don't suck water from sponges in the wild, there aren't any sponges just sitting around for crabs to drink from, they drink from puddles, they actually can't suck anyway, they just lick the water off the sponge, so you can take it out if you want to do the salt water thing. And good luck with your new crabs. Keep me updated.