I'm no professional but I have read posts before with people who have the same problem as you and changing your transmission fluid has helped others. Your supposed to change the transmission fluid at 60,000 miles if your over that mark. Here is something I pulled from a website.

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Shutting off the engine caused the transmission's fluid pressure to drop to zero. That allowed the transmission's valves to release. When you started the car again, the valves went into their proper positions.
This action can occur when the transmission fluid is low. Check the level to be certain it is filled to the proper level. Use the relevant mark on the dip stick (Hot or Cold marking) when checking the fluid level.
Very dark transmission oil may mean that it should be changed. Check your records to see if the mileage and driving conditions exceed the manufacturer's recommendation.
Dirty transmission fluid will affect, and even impede shifting. It will cause extra transmission wear.
The transmission fluid should be maintained to the proper level and not allowed to get excessively dirty. Both of these conditions can cause the oil to overheat and give it that burnt odor. This can damage the transmission permanently."

Why does my automatic transmission sometimes pops out of gear?