It sure sounds to me like you have one of two problems. Either you have something obstructing the fuel intake ... clogged filter, or gunk in the carb, or there is some type of air leak in there. Double or triple check all of your connections, make sure your filters are all clean, make sure there are no pin holes in any lines.

I have an Irvine .40 that drove me crazy with this type of problem and I found that there was an air leak between the carb and the manifold. Some high temp RTV sealant around the carb and all problems cleared up.

You may have more help if you give the details of the engine.

Just my 2 cents ... Bo