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Pulled this out of the FAQ on this site. Seems you should think about system flush and fluid change. Filter could be clogged.
A/T Fluid Needs Changing; Late or Poor Shift Quality.
Delayed Transmission Engagement When Shifting into Gear:
[Query:] The drive gear engages late when shifting from P to D in my auto transmission. [Response: Marc] The problem you describe can be attributed to either a low level of transmission fluid or a stuck valve body. If the fluid is low in the torque converter, it will take additional time to transfer the engine power to the transmission, as the power is transmitted through a fluid by spinning up a plate with fins on one side and the fluid spinning up a secondary plate with fins on the other (thus keeping fast changes in the engine power output from damaging the transmission).
I would recommend that, if you have not recently (within the last 6 months) changed the transmission fluid and transmission filter, you have this done. In my area, the change runs as low as $49.99 US, including parts & labor. If you have the Haynes manual for your car, take it with you if go to anyone other than the dealer, as the fluid may have to be drained in a non-standard way via a transmission fluid cooler return pipe (non-standard compared to other brands of vehicle). This service will also clear up most sticky valve bodies, as the new fluid reliquifies old gummy deposits...[Editor's note: see also Fluid Flush]
Late or Poor Shift Quality While in Gear:
[Symptoms:] Late or poor shift quality. [Response 1:] Since this is an unknown as to when the transmission was serviced I would recommend a power flush. Wynn's/Kendall has a machine that connects to the line to the cooler. Then they add a detergent and run the car for about 20 minutes with it off the floor and in different gears. Then they go from a recirculation mode to a change mode and add new fluid while discarding the old. This gives a full change including the torque converter. It will cost from $60 to $95 but I think it is well worth it about every 100000 miles with normal change in between. I think both my ZF and AW worked better and smoother afterwards. Call around and you should be able to find some shop that does a power flush.
[Response 2:] How dirty was the fluid was when the transmission was finally serviced? Your transmission has no bands, just clutches. When pressures are right for a shift, fluid pressure is directed to the clutch(es) that is/are to lock up. If there is a lot of clearance due to wear in the clutch packs, you usually get a delayed and hard shift. If the valve body has a problem, it could cause reduced pressure to go to the clutch pack, causing a slip as it shifts. The most common problem is governor pressure loss due to a worn output shaft bearing. Even after the output shaft bushing is replaced, the problem could still exist because while the bushing was bad, excessive wear to the transmission case where the shaft goes through, is common. A pressure test will in most cases will pinpoint the problem. This is reason # 71 for servicing the transmission at normal intervals. Every 20,000 miles is recommended. It's pressure test time.
Intermittent Shift Failure: Clogged Filter
[Query:] After starting, everything goes well, the transmission shifts, but in a short while, suddenly, the transmission becomes disconnected, losing traction; moving, I accelerate and the motor increases revolutions but the car behaves as though it were in neutral. I must stop the motor, wait a moment and repeat the operation. While the problem is occurring, if I accelerate in neutral I hear a slight buzzing noise of gears even though the transmission has not engaged. The oil is new. I changed the kickdown cable. [Response: Abe Crombie] The things you list sound like a stopped up filter inside transmission pan. Did the pan get removed and the filter inspected? The filter is a fine metal mesh strainer and can be cleaned in most cases. I didn't read your previous post of a month ago so I do not know how this started but using shop clothes to wipe off things inside transmission or to wipe the pan when it is off, can introduce lint that the transmission filter will catch when it is running. The debris on filter then starves the transmission pump for oil. The transmission pump will whine when operating with excessive vacuum on its inlet due to a plugged filter. When you stop and shut down engine, the lint falls off the filter and it will work again for a period of time until the lint is sucked up onto filter once again.
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