Pontiac Repair: transmission problem after fluid change, 1991 pontiac 6000, dextron 3

Question
I just bought a 1991 pontiac 6000 3.3l automatic with 130k on it.  Yesterday my mechanic just changed the transaxle fluid using dextron 3 and used 6.5 quarts, which brought it up to the full mark.  Today the car falls out of drive when I stop for light and I have to shift to neutral then back to drive to re-engage it, otherwise it just acts like its in neutral even though it is in drive.

I called the local Pontiac dealer and he said "sounds like it is just sucking air" and that my fluid is surely low, but now after adding one pint it is about 1/4" above the dipstick full line (using the owners manual method of checking the level - fluid hot, shift to each gear, on flat surface, etc) and it still falls out of gear at stop signs.  If I am real patient with it, it will kick into gear after 20 seconds or so. It was perfect up until yesterday, when we changed the fluid and filter.

Answer
Hi
I am not really a transmission tech, but did the mechanic replace the filter also?
If so, you may want to remove the pan and make sure the filter is seated properly and or the filter seal is is NOT cut or damaged.
Here are other items to consider.
Fluid Level
Low: correct

Vacuum Line
Pinched, slowing vacuum response

Modulator
Damaged

Oil Pressure
low pressure

Modulator Valve
Stuck or binding

Forward Servo Assembly
Damaged or missing cushion spring or retainer

Forward Servo Piston
Damaged seal

Oil Filter
Plugged

Filter Seal
Cut or damaged

Forward Servo Seal
Leaking, damaged, or missing seal
Low torque of bolts

Torque Converter
Stator roller clutch not holding

Bolt
Low torque allowing leakage for driven sprocket support

Input Clutch Assembly
Leaks at ball capsule or seals
Damaged seals

Pump Assembly
Sticking slide
Leak at the slide seals

1-2 Shift Solenoid, TCC solenoid
Malfunctioned or mislocated

I would REALLY check that filter /filter seal.
Good Luck
Pawl