Check the caliper guide pins (make sure they are rust free & fully lubed).
Are any of the anti-rattle clips on the pads missing? If so it might be possible that the pad is getting lodged in the caliper bracket at an angle causing the pad to remain up against the disc.
Based on the intermittant/reoccuring symptoms you mentioned, I would guess that it might also be something with your master cylinder/brake lines which causes the pads to not release when they should.