90' Century Service Engine Soon Light ON

I have a 90' Buick Century and the service engine light is on. It goes on and off at times but when it is on my car wants to cut off when i stop if i don't keep my foot on the gas. Do I treat my gas or could it be something else. Please help me! Thank you all for your time.

Take a paperclip and insert the ends into the two upper right terminals on the diagnostic port under the dash. Turn the car on but don't start. The check engine light will flash codes. The codes will all have two digits and will repeat three times. The first code should read code 12, then any other codes that it has stored. When it reads code 12 again then it is finished reading. You can either bring the codes to a parts store or list them here and someone can point you in the right direction.