Todd,
I am delurking again, I posted before telling you that I had the exact same
problem.  My soluiton is definitely anecdotal, but I solved the problem by being
very fastidious about making sure that my greens or any produce I eat raw are
thoroughly washed with soapy water and rinsed.  Unfortunately I do not have any
scientific references to back up the claim that pesticide residues may be
causing the problem.  My solution was a suggestion from my sister-in-law that
grew up in a largely agricultural area of Indiana.  They had to thoroughly wash
all garden produce even if it was organically grown so as not to have the same
problems from the pesticide drift from the neighboring fields.  I get a similar
reaction with things like melons, which like the greens and spinach contain
heavy doses of pesticides.  But the extra trouble of washing and watching what I
eat at those rare times I eat out has completely solved my problem.

Namaste,
Cindy