I have another theory that you might want to look into. As cars get
older they go thru many many sets of tires. Over the years the tire man
that mounts these many sets of tires slaps some type of goo on that is
supposed to seal the tire to the rim. After a while the tire change
machine scrapes and gouges the rims where the bead is supposed to seal.
Rust forms around the bead and little bits of tire bead rubber all stick
to this goo forming air gaps just big enough to let the air out slowly.
Next time you have the tires off, sand the rim where the tire seals to
get all of the chunkies off. Then goo it up and mount the tire. This may
not be the answer to your problem but it solved a lot of mine. Hope this
helps.