Sounds awfully familiar. I bought mine about a month ago after it sat up for 2-3 years. The tranny slipped a good bit when I bought it so the first thing I did was drain the fluid and replace the filter. Speaking of "the bitch", getting under there with my 280 pound body was some test of faith and love in TLC's. Anyway, after that, I poured in about 7 quarts of transmission oil and the first day or so, it ran worse than when I first got it. Then, all of a sudden it started working great. After running it for about a week, I parked it for a week and a half, pulled her out and now I only have 1st and 2nd gear and no reverse. It's the strangest thing considering where I was, what I got it to and where I'm at now.
Maybe I'm not warming it up enough? How far or long do I have to drive before it's "warm". Now I do have some leaks under there from the transmission. She's about to go to the transmission shop. Good luck and post up what you find.