A long time ago, I recall a discussion about this somewhere online (but not necessarily on this forum) where I believe the consensus was either low humidity or inconsistent humidity being a probable cause. I will be curious to see what folks say this time around.
Having said that, have the affected plants been subjected to low humidity or had periods of time when there were dramatic shifts in humidity?
And then there's the usual suspect, water. Do you keep water off the leaves? If water is getting on the leaves, is the water you're using particularly cold (or hot)?