Agreed. Sounds like a dirty comm or hung brushes. Under light or no load the motor will still spin but under heavy load it just gives up.
Is the motor or ESC getting hot? That might explain why it works after setting a while - try a smaller pinion.
It could also be a dirty connection. If the stock white Tamiya connector gets hot while running it'll have so much resistance that you'll lose power. I've actually had them get so hot they melted together. Switch to Deans or Traxxas connectors if that's the case.
GV2 Pro, XXX4G+, Blitz SCT, CRC GenXL