I have no problem with picking notes at high speed but fingering scales and the like is a nightmare. So my rhythm ability far outstrips my lead. No matter what practice I do, I don't seem to be able to get above 4 notes per beat at 130bpm if each note has to be separately fretted. My left hand can't keep up with my right. What exercises do people use to build up left hand speed, skill, and stamina?