Re: 95 Grand Marquis (weird overdrive problem)
Did they say their reader can pick up trans codes? Either way, if the trans hasn't been serviced in awhile, it would be a good thing to do. The other thought is that if there is a part in the trans pan has to be changed, the oil will be drained again. If you know of a reputable shop that can check the code for sure, that might be a good idea.
Since it's random, a trans service may fix it. That model has a drain plug in the torque convertor that can be drained also. It takes about 10½ quarts to fill it then.