Case 680C Shuttle Problem

My 680C has developed a slipping problem when it is in forward gear only. When I shift into forward the shuttle is slipping from fully engaged which is normal to almost into neutral. Notice I said almost neutral. It does this at constant intervals. I can better describe it as chugging effect. It will do it with any of the four gears in the four speed transmission. It is most severe when the I first start using it with the machine at air temperature. The severity decreases to almost unnoticeable at operating temperature. The oil in the shuttle and the convertor were completely changed 300 hours ago. The oil filter (screen) was spotless at that time. I changed the oil in the shuttle last week and it was clean and again the screen is spotless. I couldn't change the convertor oil without removing the shuttle. Yesterday the machine was at operating temperature and I locked the brakes, put the gear box in fourth gear forward and with a wide open throttle I could barely feel the shuttle slipping symptom. It did it at constant unchanging short intervals. The convertor seems to be in good condition because the engine was under a heavy load. The convertor temperature barely moved up. I only did this long enough to determine that it was doing this with a heavily loaded drive line. Would anyone have an idea as to what is causing this ?