Seedling problem

Once in while when starting seedlings in 8 oz. cups with organic potting mix, the seedling's main stalk will wither to the size of a hair just above the soil line and the seedling falls over and dies. The problem occurs over about a weeks time. This doesn't happen with every batch of seeds, just occasionally, but I hate to lose any at all. When the seedling dies and is remove from the cup the root also has shriveled up. Anyone have an idea what is causing this?

It could be a few things (And I'm sure more than I thought of )

Don't feed it nutes when it's that young, you'll burn the roots since they are so small.
Make sure to keep it misted but not soaked, make those roots work a little bit to find their moisture, it makes them grow stronger and bigger.
Have some light air blowing on it so the stem gets strong

Good luck, it's very frustrating to pop a seed and not get to see the baby grow!!

i havent grown from seed since high school and i seem to have a really good outcome this time around, i just use fox farm ocean forest soil with NO nutrients at all in a 8 ounce (i think) red dixie cup with holes drilled at the bottom, i keep a fold top sandwich bag over the seedlings and they are about 12-14 inches away from a metal halide 400 watter at about 50% humidity in the cabninet (probably 100% humidity in the bags they are covered with. temps are around 79 and growth is strong and fast,

click on my signature if u wanna see what i mean, id be more than happy to have u guys swing by and check it out anyways....