What would cause an RPM drop and engine cutoff after starting a Cessna 152?
Gradual power loss would not be caused by "air in the fuel lines".

The most likely cause is blocked vents. There are pipes that allow air into the tanks so the fuel will flow out. If they are blocked for any reason, fuel will stop flowing, often in a gradual manner.

Another possibility is that there is corrosion or sediment which is blocking the fuel line. Are you checking drain valves on every preflight and after refueling?