Snapping and alignment

Why does everytime I try to line up a large piece to another large piece does the dang thing snap while I am in manual movement mode? I mean ok I get it the snap system is either not finished or trash. However, if I am in manual move mode I should be able to look a part and rotate to the position I need but it gets close then suddenly bounces to some weird angle. If I am in manual move it should float in the air not suddenly snap to some part. This issue is especially acute when aligning triangular pieces at roof corner turns.