I have a 86 El Camino with a 305 and automatic (99,000 miles 2nd owner)and usually after the car is warmed up it will shutter when slowing down for a light or turn.. usually
around 10 MPH approx.
I have had the transmission flushed because I thought that would help but it didn't. If I put the car in Neutral or if I'm going slow enough I can hit the brake harder and it stops. and it does it more then it doesn't.

any ideas would be appreciated

Does that transmission have a lockup torque converter? Shutterin can be a symptom of the converter NOT unlocking properly. If so you might have it flushed again??? Does it happen all the time or only when slowing from higher speeds?

Are you saying that hitting the brakes harder stops the shuddering? If so, you have brake problems.

If it shudders when the brakes are applied you have discs/drums with excessive runout. Do you feel the pulsation in the brake pedal? If the car shudders when slowing down from higher speeds with the trans in high gear it could be the lockup converter not disengaging. The switch at the brake pedal, solenoid in the trans pan or clutches in the converter are defective/out of adjustment.