1997 Chevy Cavalier Crankshaft Position Sensor

We had a new computer put in car due to Codes showing up at emissions, new fuel injectors and overall tuneup of the car. The check engine light was on. After all this was fixed we were told to have the crankshaft sensor "relearned". When we first got it back from the mechanic after all afore mentioned work, it worked great and check engine light was off. Then a day later, check engine light comes back on and the stalling started again. After a month the battery goes dead, we replace and then the car could not be started with out a jump. Then the gas pedal had to CONTINUOUSLY be pressed or it would stall out and would not start. We took it to chevy dealer had had crankshaft sensor "relearned" and the check engine light went off and it drove like a champ turned over no problem and smooth sailing. For about 3 hours. Now its back to stalling unless gas is pressed. It does however start up without a jump now. PLEASE HELP this is my daughters car and I'm afraid for her safety. Any info is greatly appreciated. (The codes it had for emission failure was P0336 crankshaft sensor not learned. SOmthing like that if that helps.