I have a mid 90s Walker MC. Today, when I went to cut the grass, the mower started, I engaged the blades, and went about 20ft into the lawn, and the blades dissengaged. The mower did not stop running, just the blades. I went back to the driveway, the blades came back on by themselves. Back into the lawn, the blades quit. After awhile of this, I could not get the blades to engage. I pulled the wires off the PTO switch and put them on again. Blades then engaged, for about 20ft then turned off. Occasionally they would try to come back on. What is the most likely cause for this problem. Thanks to you Walker gurus.