Mitsubishi Repair: 90 mitsubishi galant 2.0L, mitsubishi galant, gas mileage

Question
Symptoms: cars stalls alot as if it were running out of gas, black smoke comes out of tailpipe, rpm needle is unstable rising then lowering til it shuts off. It used to shake alot. gave a tune up. replaced lifters, one was bad.found spark plugs very black- like soot they were changed again. now only shuts off when at complete stop. friend suggested that the cat might not be working properly.(exhaust system bad?)
Please help, Thank Q  

Answer
S.,
Sounds like your car is running too rich.  That's indicated by  the black soot on your spark plug and black smoke from the tailpipe.  Running rich means you're burning too much fuel.  You may have noticed a reduction in gas mileage.  

I can see two possible problems.  One, the unstable RPM needle indicates a malfunctioning Idle Air Control (IAC) motor.  This is also referred to as a Idle Speed Control (ISC) motor.  The IAC's job is to regulate the idle speed when the car comes to a stop.  When it starts to malfuction, the IAC just guesses where the idle should be.  This is commonly called "idle surge".  You'll want to have your mechanic inspect your IAC motor to confirm.  The second problem you may have is a fouled Oxygen (O2) Sensor.  The O2 sensor reads the amount of oxygen passing through your exhaust.  When the O2 sensor gets fouled and malfunctions, it always assumes too much air is entering the engine and increases the amount of fuel.  Hence, too much fuel enters, causes black soots on your spark plugs and black smoke comes out of your exhaust.  Have your mechanic check your O2 sensor as well.
Good luck!