92 chevy falling on face
 

I have a 92 chevy Cheyenne with a 350 and 4l60 has a mild even rv cam and headers otherwise stock it was not shifting till it go to 35mph I changed fluid and filter it shifts better but now it started falling on it's face when it hit 55 started this this morning and by tonight it has gone to backfiring and falling on its face at 20mph and seems to go in to neutral and reving up just before loosing power and backfiring what could be the problem