pc200-6 wont rev out after half an hour

Good evening all , got an old pc200-6 farm excavator . after running fine for 20 minutes -1/2 an hour it gradually wont rev under load . After idling for a few seconds it will rev up but then the problem returns . After being turned off for a few houres its good agin for 20min/half hour then again wont rev --- thanks ---mat --- :-)

pc200-6 wont rev out after half an hour