I have a John Deere LX 280 Garden Tractor or Mower, when I attempt to engage the mower blades I lose power and even shut down the mower ( once) when I disengage the mower blade switch it gains power again.
Remove the deck belt from the engine pulley and try spinning the deck spindles. A bearing could be seized up or you could have grass built up under the deck interfering with the blades.