First thing I would it try applying a little choke while under load. This will verify that if it is lean out problem as it will run better if it is. I would also do the basics like installing a known good plug as they do fail under load, adjust the valves, and clean the carburetor. I recently had a GX390 on a pressure washer where the plug gap was nearly .060", it couldn't even run under load without stalling out.
Might wonder why I suggest cleaning the carburetor. Here it is common for carburetors to clog up and just saves time to go ahead and do it.
