I have had the adjusting nut back off on cars before. Adjust the play and mark with a sharpie and see if it is moving.
Then if it is, come up with a way to secure it. Double nut, new nylon nut, etc.
You could even have a problem with the nut pulling through the rubber insert in the fork eye. Make sure you have a nice thick washer of some sort between the rubber bushing and the adjusting nut. I left one out on my car this past year as it sat for several years and parts got moved. It worked good for awhile and then pulled through the rubber.
I'm pretty sure your car is losing the adjustment if it works good for awhile then starts having the pedal not return.
I bet when it happens it returns a little bit but not all the way, correct? Have to pull it on up the last couple of inches with your foot?