[Towncar]

Sounds like a vacuum leak somewhere, maybe the main vacuum line to the brake booster.
Is the problem just when the brake pedal is applied? Try it in D or R with the parking brake locked rather than the brake pedal to see if the brake booster is causing the problem.

Does it idle worse when you pump the brake pedal? The brake booster consumes vacuum when actuated repetedly.

Other than vacuum, normal wear on spark components will cause the problem you describe. If you havn't replaced your spark wires and other high voltage parts for awhile it's a good time to do so.
You need to rule out the obvious and most likely causes before moving on to the more expensive and difficult to diagnose parts/causes.