The fact that the symptoms keep returning lead me to believe it is a problem with some kind of buildup on the rotor face(s). The leaking caliper may be allowing fluid onto the face (the caliper has been leaking for a LONG time if this is the case). Maybe the way you use the brakes is causing a quick buildup of pad material.
Most of the other suggestions I've seen in this thread would be immediately apparent and not take a while to show up.
YMMV
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