I had similar problem with my OS .60 w/pump. To really find out if it's a pump problem, try this first. Clean out the carb. ( Keep your fuel line attached to the pump and take the output line and have it discharge in to another fuel tank so you can observe the fuel output). Then using a separate tank, hook it directly to the carb with your fuel line.Fill both tanks with fresh fuel. ( Set your tank level or above the carb, with or without  muffler preasure.) And adjust the carb as if it were a non-pump engine. While the engine is running now you can observe the fuel flow at all speeds. If it runs ok, the problem could be between fuel tank to the pump.