Sounds like maybe your radiator cap is opening at too low a pressure. If it is operating properly, the radiator cap should not open to the overflow unless the pressure exceeds 15 psi.
I would start by having your radiator cap checked to be sure it is still holding rated pressure (15 psi) without leaking down. If you don't have the tester in your tool box to do this, any radiator shop can do this for you. Just takes a couple of minutes. All you have to do is bring the radiator cap to the shop for the test.
Had a similar problem on my ride a couple years ago - new radiator cap and all was well.