Turned out to be a bad coil pack in my situation. My mower would do it hot or cold, just started it 1 day out of blue. Start & run fine, all power it world. Then engage blades (engine revved or not) & engine would start bogging down. Turn blades off, revs back up by self. All spindles loose & well greased w/moly grease as usual. Went through uel system, plugs & finally found w/infrared temp gauge R bank shutting down. Changed coil pack, all good. Changed other after that just to make sure good. Runs fine now. 12 yr old Simplicity Cobalt 61” ZTR w/non ICD deck & 27 hp Kohler Non/FI. Mows 6 acres a wk. Have a good day.