Tranmission disengage from Drive after stopping at lights

My 1997 Sable wagon with Duratec engine will occasionally the transmission disengage (5 times already) from Drive after stopping for the traffic lights. There are two ways to have the transmission engaged again; 1)move the shifter to Neutral and then back to Drive. 2)tap the gas pedal a few times and wait until it engages again. The transmission fluid level is more than sufficient. I will try to fix it myself before sending to the mechanic. Can anyone advise me what is going on and how to fix the problem. It is a bit dangerous to have the car not moving at intersections.

The transmission fluid level is more than sufficient

Well having it right, might be better.  Do you mean it is overfilled? or low, but not so low that you feel it should be a problem?  How many miles on that Sable and how many miles has it been since the transmission fluid was changed?

when it first started acting, I was worry and added some fluid to make sure it was enough. Later on, I found it became a bit on the high side when warm. I am too lazy to suck it back out. The mileage on the car is ~125,000. I changed the fluid and the filter at ~98,000mi and is scheduled to change transmission fluid at 130,000. The fluid still look good and no burnt smell.