Shift solenoid error causing misfire

While I was driving one day my lumina started acting funny as in loss of power and using excessive gas then my check engine light came on and my engine starting to misfire only three of the six are firing. I have checked for shorts in all plug wires and also brought it to a computer and it says "Shift solenoid electrical circuit A" when i delete the error from the computer log CAR RUNS FINE for about twenty miles then light comes on again and starts misfiring again PLEASE HELP