Floury potatoes are not great for boiling, they break up easily and don't keep their shape.
Boil them in their jackets. That's the traditional Irish way, and we are the land of the floury potato :)
not cooked enough, or it gets burnt
Here's a few tips, if you haven't already tried them.
Use a glass dish/plate. It conducts the heat differently and avoids a soggy base.
Oven temp around 170 degrees, middle shelf, longer slower cooking is better.
Make sure the pastry is really cold when it goes in the oven. Make the pie/tart or whatever a couple of hours ahead, pop in the fridge and then it'll be nice and cold going into the oven.