What happened to the thrips diagnosis? That seemed reasonably
likely to me. According to this page,
http://pubs.ext.vt.edu/444/444-281/444-281.html
thrips are attracted to flower pollen, but feed mostly on vegetation
rather than pollen. If that vegetation is in your flowers (pistil, style,
anthers, etc), I would expect that to lead to blossom drop.
You can find out if you have a thrip infestation with blue sticky traps.
Unfortunately, most of the biocontrols for thrips listed on that page
seem to operate best when humidity is 60% or higher, which would
be pretty rare at this time of year in your part of the country.