Hate to chime in on a post to latewood, but I couldn't resiste putting in my two cents worth.

After reading your problem, my first gutt instinct is that somehow those plants that wilted so quickly were starved of water. Here is my reasoning....

If this was a PPM problem, then it's likely that you'd have seen some indications of over/under fertilization from the plants leaves long before it completely wilts.

If this were an oxygen problem, that too should have shown up as a deficiency prior to wilt. Assuming that you had "some" air present in the water.

Water starvation can be caused from lack of exposure to water itself (clogged system), or the inability of the plant to uptake water (pythium). If the roots are suffering from pythium, enough to wilt/kill the plant, then you should be able to detect the odor on the roots. It appears you have everything under control as far as temps/PH/nutes (they don't appear to be too extreem for any grow) so I can't help but believe that the roots are the 'root' of your problem.

When you said the leaves were toasted, did you mean that they looked burnt (spotted, rusty, cooked from heat) or that they were wilted down to a stringy looking green leaf (like cooked spinich)? If they were just wilted and stringy, then that is just the same reaction the stem exhibited due to lack of water pressure in the plant. Soaking the plant in air saturated water should bring it back to life within hours if it's just water starved. If it's pythium, you'll need the power of God to bring it back.