Sounds like a rebld may be in order. Has the fluid been changed in it ever? IF you have the money get a tranny flush, see if that helps. A flush runs about 100 bucks. Rebuild is about 1400 or so i think.
70 Camaro SS // 87 Mustang GT Conv - 119k
93 Z71 - 264k // 01 P71 - 87k // 05 Dodge Magnum R/T - 122k