For a while now, I have been having issues with some projected curves. Some parts of the curve just seem to veer off, without obvious reason. I have included an example of one such case. No matter what I try, I cannot get the curve to behave. I have made sure both sketched lines are absolutely horizontal, as is the attaching bit of the adjacent curve. They both are on the same plane. I also have tried different types of splines and putting the curves on parallel, but different planes. The result is always unpredictable and unusable. No matter what, the erratic variance in the curve shown always seems to occur.

What am I overlooking? Why are my projected curves behaving like this?