from all that has been said, my pee brain says you have some kinda dirt or object floaitng around in the line, maybe its hung up at the proportioning valve. when it floats over just the right way everything is good, when it floats the wrong way and clogs somthign up a bit things go haywire.
If it was me, I would disconnect every line one by one and blow them out then flush new fluid all the way back through and while you are at it pull the porptioning valve and check it over rplace or whatever, and when the valve is out I would blow that part of the lines out as well...
I would then put it all back together and fill and bleed it with cheepo fuild a few times, then flush it through with good fluid before taking it to a track day, I wouldnt want to show up at a race with unknown brakes.
BTW I had a similar issue this spring at the Drivers Ed at cota, I had Gilfus swap the master to a new one and bleed, it was better but not great, it has gotten me in a great habbit of always checking those brakes before the brake zone, the problem has pretty much worked itself out now, but I notice it comes back a bit when my pads are low. I nevr had this issue before we rebuilt the car over the winter, we had all brakes off and subframes replaced etc. I really think we had some dirt in the lines that was playing havoc with the system, i think it has wrked its way through on my system, but I have not been on Gators since Cota, regualr curbs dont knock my pads back like tha gators.