Can you explain further what you mean about not geo-referenced.
Personally, I've never found a real use for the A+B line function of most GPS guidance systems. Everything we do is A+heading. I'm always surprised when the units default to A+B when defining new swathes. I guess maybe it's because our fields are all circles (or squares surveyed on a grid) and we tend to work them straight north and south (or on a well-defined angle like 345 or 15) to make it easy to get around the pivot. We always start to one side of the center line of the field for every operation.
The surface of the earth is curved, whether you can see it or not. Your guidance lines being non-georeferenced means that they do not conform to the curvature of the earth. Therefore, it's like putting a square metal grid onto a curved surface. No matter how hard you try, as you get farther from your initial guidance line (A+, AB etc) your accuracy degrades and you will notice line shift.
I always tell my customers to name each and every field, be it a quarter section, half section, whatever. And have a separate guidance line for each field. Setting new A+ lines in the Pro700 is extremely simple, and don't worry about filling up the memory. Guidance lines and field names take up very little data compared to prescriptions etc.
The majority of my line-shift complaints come from guys who run one or two guidance lines all year and just make a new one when the display tells them they have to. I instruct them to create a new line and the problem goes away.
80 passes isn't a big deal (60ft passes x 80 is less than a mile) but you are on 330. That is a big deal. Do not record your A+ line unless you are in the field, and create/select a guidance line for each and every field you work in and I can assure you that the line-shift problem will be alleviated.