Tactics Trainer

My scheme for Tactics Trainer is as follows: to take as much time as necessary to solve the problem even if that means getting the minimum score of 20% and a reduction in my grading; to play each problem through twice, so I can absorb its lesson.  I want to maintain a grading above 1700.  My grading has fluctuated a lot and I'm puzzled by this as my grading in other activities is fairly stable, except where I dropped 150 points in my Online grading following a decision to withdraw from several tournaments at once when I found myself over-committed.