Surging

My aging sister is experincing SURGING upon slight accelleration afler being in high, (the last automatic shift), gear in a 97 Continental with 46 thousand miles.
Fuel problems have been eliminated by going to a next lower gear while driving and NO surging is present at the higher engine RPMs.
I forgot to check for the surging while in the next lower gear.

Could it be a problem with "AOD", (as I have read under "funny transmission"), if this vehicle has it?

Has anybody else experienced this, and know the cure, before my sister goes to the DEALER?

Thanks, from the ("Visiting" Brother)

PS. I,m very new at this "posting" so forgive me of any mistakes!