At the MARA Conference, DEP gravel road expert John Maclaine recommended crushed bluestone, well packed, as being a good solution for problematic stretches of road. Also, of course, there is the importance of good drainage.
Does that section of road have sufficient ditching so the the water has somewhere to travel other than down the road surface? If there isn't enough width to provide sufficient ditching, or if there isn't enough depth of material to create ditches without hitting ledge, It's possible that the best you can do is build up the road and crown it well so the water at least stays to the edges of the road. A "broad-based dip" above the top of the hill can help limit how much water goes down the hill from the top. Also, if you can put in turnouts anywhere, that can help reduce the amount of water that runs down the hill. Reducing the volume of runoff will help reduce erosion.
Our road has sections where there is ledge just a few inches down, so ditching isn't an option. We have just added material to the road to raise the crown, and have cleared out a couple of turnouts besides the one above the broad based dip at the top. Hopefully that will improve the situation.