My 2006 RTX50 (model 17AA5D7P710) with Kawasaki engine has developed a frustrating problem. When the mower is under a load, the engine starts stumbling-cutting out. I noticed that there didn't seem to be any fuel in the filter. I replaced the fuel pump and all the fuel lines after draining and cleaning the tank. Now there is fuel in the filter at all times even when engine is stumbling. I swapped the carburetor with the original, which I had replaced to troubleshoot a hard starting problem. (It didn't fix that issue, so I believe the carb is good.) It seems to run all right unless it is under load. While it is stumbling, there is no evidence of smoke indicating running rich. My next step is to start throwing money at it, ignition modules, etc. I would rather not do that. Anyone run into this problem before? I would appreciate any help.