That is some fine work there my friend. My father clears power line right-of-ways for utility companies. He's been in business for 43 years and has had numerous "factory" stump grinders. Of course growing up, during the summers, I was fortunate enough to "get" to grind stumps all the time. In looking at your grinder I was wondering how you move the grinder rearward as you grind away on the stump. Do you actually have to back the tractor up? Factory units are built where the trailer tongue is actually two pieces (one insided the other) with a cylinder that will literally lenthen the tongue. That keeps you from having to back the truck up an inch or so every time you make a pass. The beauty of moving the grinder rearward with hydraulics is that you can literally move it back the cutting width of the tooth. Much more precise than trying to back a tractor or truck.
Also, if the cutter is wanting to "walk" over the top of the stump you may just be trying to
cut to low on the wheel. Rather the
cut the stump with the, say 5 o'clock part of the wheel, drop it down a little and hit the stump with the 3 o'clock part of the wheel. Not sure if this will help, but worth trying. I wouldn't take much of a bigger bite than that or the wheel ends up having to drag the
cut nearly 180 degrees and that will really tend to bog things down.
Just a few thoughts from someone who has spent many hours grinding many stumps over the years.
Again, that is an excellent looking machine. I'm sure you will enjoy it more and more as you learn to use it better. I would love to build something like that. Stump grinders also make fairly good trenchers.