79 case 50 acts like it bogs sometimes

hi guys i have aquired a 1979 case 580 backhoe and its developed this wierd problem, every one and a while randomly i will go to shift it in to gear and it will bog down and wont rev up, i will have to shift it back into neutral and then rev it up and then go back into forward/ reverse. after that it wont be a problem again for a while. any thoughts would be greatly appreciated