Allis D-17 problem

I have a nice model d-17 model 4.  I have never really had any major issues until this spring.  It sat in the shed all winter and I went out in March and it fired right up.  She will run perfect for about 30 minutes or so with full power, after that she dies down and will stall when put under any kind of load.  She is barely able to move at all without having throttle all the way open.  I notice that after the tractor gets warm she is leaking some oil.  I first thought just the oil pan gasket was leaking but after further inspection it seems like the oil is leaking farther back under the operators station.