Transmission slips in Overdrive. (Only) O.D.

Just bought this 93 T-Bird. on ride home car had no O.D. gear  just slipped (notta) examined vehicle at home, found low trans fluid.  Ok , easy enough, added some still slips. Following morning checked fluid again appered low still, had Wife add more now it's a quart over full. (Whoops) Next day noticed kick down cable was off.  Easy also cool ! (NOT)  Still slips in O.D. Tried to adjust kickdown cable, Maybe i did it wrong. Please explain if you know how. Could it be couse trans is a qt. over full ?  Thought about changing filter & fluid & getting fluid to correct level.  Or am i just wasting my time... Really don't want to do a Tranny job !!!!  Can any one please help me with some Info. Would really appreciate it.                                                                                             Thanks Chopper