Automatic - Third Gear comes and goes?
 

1993 300ZX 2+2 NA automatic Trans
If I accelerate (pulling) hard in Third Gear, then “let-up” on the gas pedal, stopping the car accelerating, third gear is gone. If I accelerating hard then “left-off” the gas pedal, letting the car coast down (MPH) then applying the gas pedal again, third gear is back. First, Second and OD no problem just third gear. Any idea what going on?