I deal with this problem day in and day out . I can help you,\
First Check your Fluid Level !! if its ok ,
You Either have a Drainback problem, or an Internal problem,
First , The Drainback problem if the Transmission fluid is allowed to Drain from the Torque converter while sitting for a long period It could be the Anti drainback Valve located in the Lower cooler line where it goes into the case of the trans, This can be replaced easily, also the converter Bushing and or Stator hub (internally) could have excessive clearance to allow this drainback.
To see if this is the problem , Check your fluid level running and warm, Make a mental note of this Then While Cold in the morning BEFORE you start the car check the fluid level, It should NOT be any higher than 1half of an inch higher than your Warm level mark , If so You Have a Drain back problem.
If its within this limit, Unfourtanally the Input (foward ) Lip seal is hard (internal) and will need to be replaced .
it will get worse as time progresses but it will NOT Quit on you
You see when the oil fills your Foward clutch everything works,
and it will continue to work Just that it may get slowler as time passes (month`s)
it`s just that the oil take`s a little longer to get where it needs to go