1998 Mercury Sable Engine Surges

Could be one of the following below:

Dirty fuel injectors (cleaning the injectors often fixes this).
Bad MAP (manifold absolute pressure) sensor
Bad TPS (throttle position) sensor
Bad or dirty MAF (mass airflow) sensor
Low fuel pressure (leaky fuel pressure regulator or weak fuel pump)
Vacuum leaks (intake manifold, vacuum hoses, throttle body, EGR valve)
Bad gasoline (fuel contaminated with water or too much alcohol)

Sometimes, what feels like a hesitation is actually ignition misfire rather than lean misfire. The causes of ignition misfire may include:

Dirty or worn spark plugs
Bad plug wires
Weak ignition coil
Wet plug wires

Before you get all crazy with stuff, do one simple thing first.

REPLACE your FUEL Filter. The ethanol in gasoline tends to clean out older metal gas tanks, and it's quite likely your filter is clogged with lotsa sediment. For $12.69 at Wal-mart, it's an easy fix, and will likely solve this problem, before doing all the expensive things mentioned above. Mind you, if it doesn't then do follow those suggestions, but any garage should start with the cheapest first, and that's usually the filter.