We had a motor put in our 1993 Chevy Stepside (350 motor) 4wheel drive. Since we got it back, it does not want to shift gears when it should when going from 2nd to 3rd, and when it does finally try to shift, the motor dies. No warning...just rpm dies. Starts right back up again when put in neutral. PLEASE help! Cannot drive with this going on and have spent $1200 already! Any suggestions would be greatly appreciated.