Question: Does it only do that while in cruise control? We have had a couple of grandmarquis' that have seemd like the transmission was going out.....like you could lay it to the floor and wouldn't hardly get anywhere, engine racing, and have trouble shifting between gears. It turned out (one is an '83, other '92) that a small, $2 peice of plastic that I beleive connects some linkage between the throttle body and transmission was broken, and prevented proper shifting (and cuased excessive wear on the pressure plats in the tranny). I've heard of a guy that burned up his transmission because it was like that for too long. I also heard of a guy that replaced that small plastic part with a LEGO (lol) and worked fine. Other than this, it might be your cruise controll actuator croaking. You may just have to keep your foot on the pedal ;-)
And if that wasn't the problem......maybe you'll come accross that at some time or another and then you'll know what to do! Hope it hepled.
Nic