Why does my car cut off when i put it in gear

Transmission shift solenoid, locking up torque converter too soon causing engine to stall. (This assumes you have proper idle, if it doesn't feel like it can idle, then a throttle body cleaning may be in order.) Seafoam (or equivalent) for transmissions might help you out, after you run it for a short while with the seafoam take it in and replace the trans fluid.