In Settings>Routing, have you set Recalculate to "Off"? At the very least set it to "Prompted", not "Automatic". Automatic is a terrible setting because as soon as you deviate from a planned route, it reroutes you and you have no way of recovering your original (therefore, no way to find your way back on course).
I find that when you have a route with overlapping segments it's better to break it up. Otherwise, even if it doesn't alter your route the GPS gets confused about where on your route you are and keeps telling you to make a U-turn.
Garmin used to have older versions of the Edge 705/605 firmware available for download but unfortunately has decided to remove all but version 3.30. It's unlikely they'll continue to develop and fix bugs in the firmware as the 605 and 705 are discontinued.