Why won't my car maintain speed with out lunging and cutting out?

Hello, It sounds mostly like an issue with vacuum. You may have a leak, which starves the engine of air, and the lunging would be the recovery as the computer cuts fuel. At a constant speed, flat road, the engine should maintain vacuum nearly perfectly, but if you have a leak, the vacuum will die down at constant speeds. Also, you could have an issue with airflow/air pressure sensors, making the computer react as if there were a vacuum leak, even if there is not. Have you had your vehicle serviced recently before the issue began? Can you scan for trouble codes and reply with them?