I know exactly what you're talking about.
But I think that with a slipping auto trans, you might as well save up to have it rebuilt because I don't think any atf could bring it back. It might help for a while, but the trans itself is simply getting worse because the materials being put in there are 'courser' to help grab. So your clutches are being warn away even more if that makes sense.
All you're doing is prolonging the inevitable and one day, it will simply TOTALLY give out and leave you stranded.