It jumps around because you're not holding the sander flat on the surface. I've been working all week with such a drywall sander (no Planex unfortunately but an el-cheapo Hyundai) and I had the same at first. It sure is difficult to control the sander on the ceiling.
Because of the long neck, you need quite a bit of force to keep the pad in the right position. It becomes harder the further you have to reach. But in the end I got it under control, it is a matter of getting used to the sander and adapt your muscles and movement. If possible, don't stand straight under the pad, always work under an angle, this will give you better control. I used 120 grit to sand the first layer of drywall compound, and 180 for the second. I also found that keeping the sander on full speed worked better than dialing it down.
I was sanding freshly installed drywall, but I guess that if you're sanding a popcorn celing or something like that it is even harder due to the rough surface.
