Could a leaking vacuum brake booster be the culprit? Turning the engine off would release the vacuum, hence why power would seem to be restored.
Easy way to check - run the car until the symptom occurs, then jack it up (while still running!) and see if all the wheels spin freely. If not, brakes are locking up and you need a new booster.
-Boyo
p.s Mercury!? This is a Chevy forum!? Sorry, just had to do my moderator duties.
