Could be fluid level or filter as Paul says, could be the transmission is tired.

When a transmission's seals are so bad that the internal leaks don't allow it to build adequate control pressure, the car won't go. Heating up the fluid and seals will then make them soft enough for a decent seal. Common failure mode.

If you've exhausted all other possibilities, a temporary fix is possible, where a solvent is added to the ATF to soften the seals, I believe a small quantity of Acetone is what some of my old-school transmission guys say to use, amount unknown to me. Several ATF additives are avialble to accomplish this. It is not a permanent fix, and certainly won't do the transmission any good, but WTH, it's rebuild time anyway.

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Gone to the dark side

- Jeff