Charles,
What is the condition of the batter and charging system on this tractor? Does the gauge show a charge or discharge while the tractor is running, and with the mower running? Does it start on its own or do you have to jump start it? If it has lights, are them dim if you turn them on when the PTO stops? The PTO draws a good deal of power when it is running, if you have a weak battery or it is not charging it will not be able to supply enough current to keep the PTO engaged. I have a 1450 and with the tractor running the needle over toward the C on the gauge, with the PTO on it is closer to straight up and down, but still toward the C side....
Scott