Sometimes my car slips out of overdrive, why?

The trans in this vehicle is electronically controlled by the computer. There is no modulator. There are various sensor inputs that will cause the symptom you describe. TP, TR, TSS and VSS inputs can cause the concern, but also present other unique symptoms that differentiate one from the other as a cause. This does not necessarily mean the part is defective. It may be wiring, an internal trans component or a poor engine-to-body ground. However, to identify which sensor, it is important to have the rest of the symptoms. You are providing incomplete information for anyone to give you a specific concrete diagnosis. You are even hurting your own chances of identifying the problem by disconnecting the battery each time. By disconnecting and reconnecting the battery, you are erasing any trouble codes that might have been there to aid in diagnosis. Quit disconnecting the battery and pay attention to any other symptoms. Do not be surprised if you get no trouble codes at all when you have diagnosis performed. Some faults in this vehicle do not set trouble codes.