My girlfriend had recurring PO on her hairline several years in a row, and used applications of pure Rosemary oil to subdue it. The Rosemary caused it to go away permanently. It's an autoimmune reaction, so each person will probably react a little differently to folk treatments (like for psoriasis).
I've read that the chemical that causes the dermatitis urushial adheres itself to the innermost layer of skin and then kills the epidermis above it (causing blisters). Perhaps some remains attached and can flare due to stress/heat/etc.
