If it lifts, then drifts down and then bobs back to the original position there a couple of possibilities.
1. There is a leak in the lowering speed control valve. Just under the seat, between your legs. With the arms lowered all the way you can disassemble the
valve, inspect and clean it.
2. The seal on your loft piston is worn and allowing fluid to leak past and the arms lower. With the tractor running, the position control linkage senses the
downward movement and pumps fluid to correct for the drift..
Go to Hoye Tractor on the web. Even though they concentrate on Yanmar tractors, there are some excellent articles and videos in the Knowledge Base section. There are many similarities between the various brands of tractors made in Japan.