If the auto transmission fluid has not been changed a while, gunk may cause blockage and gear slips.
There is a transmission flushing procedure done by shops with a machine, or you can do it yourself, which really helps.
Basically flush the fluid from the pan through the transmission and fluid cooler out while adding new fluid in::
1 Jack up the car's driving wheels so they can drive.
2. Add flushing liquid to transmission, idle the car and shift through all gears for about 10 minutes
3. Drain the transmission, measure how many quarts of fluid are drained.
4. Tighten lug. Add back 4 or 5 quarts of fluid via the dip stick
5. Remove the xmission fluid hose from the radiator cooler back to the transmission so the fluid in cooler also gets flushed out. Stick the end into a big jug to collect fluid.
6. Idle car, shift through all gears while adding new fluid into dip stick.
7 Watch the fluid in the jug.. When 4 or 5 quarts are flushed out or when fluid is no longer black/brown and turns red, then the transmission is completely flushed.
8. Hook the hose back to the transmission.
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