I think it is easier if you are playing a scale than if you are playing an interval (a bigger interval than a whole step). I am not sure what the reason is though.
Of course, it is easier to play in scales than in large intervals, because the ways of playing two notes next to each other are much more alike. In order to improve the low C, you can start at a note that you can do with good sound control, then slowly step down, trying to maintain the quality of sound and volume of the first note. No need to step down from a note that you can't play well.
