My Ford 641 tractor lift will occasionally make very small incremental jerky lifts when I raise the lift lever. It comes and goes; I first noticed it maybe 5 years ago during cold winter but returned to normal operation after a warmup. It was not low on hydraulic fluid but I added some anyway. I have also noticed it in warm weather. It has done this only 3 - 4 times since the first observation so I have not done anything about it. Any hints as to cause and solution?