Engine runs hard then locks up

I have a McConnell Marc 25 hp tractor that I mow my lawn with. 'Bout the third time around it starts to run hard and as fast as I can I throttle down and the engine stops dead. Thirty seconds later it starts right up and runs fine for the rest of the lawn. The temp. never goes over 165 degrees and the oil pressure is right where it should be. It has done this probably four times. The filters and oil are fresh and when I changed it there were no metal filings in the filter. This one has me stumped.