Sounds like a trans front seal or a pan gasket. If you let the car set for weeks at a time without being started, the front seal can distort. Driving the car can restore the seals shape. When the engine is stopped all the fluid runs back to the pan and over fills it. If your pan gasket isn't sealing well, it will leak then. If these two answers aren't correct, you could have a badly worn front seal which translates to removing the trans to install a $5.50 seal.

Peter

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