I have a craftsman (Bob Villa) edition riding mower. model 917 274033?

The engine runs great when not under load. When I put the tractor in gear either foward or reverse and let out the clutch pedal the tractor dies. Push it back in and the tractor runs. Also engaging the blades will also cause the engine to die. I have checked all fuses, the dead man swithc on the seat removed and cleaned all connections in the electrial system. cleaned the air filters, and replaced the fuel filters. My electrial schematic is pretty useless. Need some help here if you would. Im pretty handy (or so I thought) I cant seem to find anything on line that can give me a clue. The engine is a briggs and straton 21HP OHV Vtwin 6 forward speeds 42 inch cut.