Trans going in to gear

I have a weird problem. I have a 727 behind my 440, and when I go from park or neutral to drive it goes in to 1st real soft then a couple seconds I feel a jerk and the car lunges forward and it feels like it is really going into gear. If I try to take off before I feel the lunge the transmission slips. I've checked the fluid it's full. It is a totally stock transmission with a stock torque converter. Give me your thoughts on what might be going on. When the car is in park the motor is ideling about 800rpm then in gear it drops down to about 600.