Ford Mustang GT No idle

Recently my 98' Mustang GT has starting to have what I believe is transmission issues it's an automatic. When I put it into drive there is no idle speed, I have to raise my RPM's to a non specific height from 1000-4000 just to get the vehicle. It runs fine after it is worked into gear, but if I floor the peddle the RPM's will free idle until I release the gas then it gives a hard shift into the next gear. I think I may have an internal crack, but I'm not sure.

Have you checked the fluid level?

Have you checked the fluid level?

Yes I have multiple times. It was full twice, then It was low about between hot, and cold on the dip stick while at operating temp. I gave it one quart, and it now it shows it is over filled. A mechanic used a bottle of seal rejuvenator, but that didn't do anything either.