Tires are not stability rated, it's a speed rating. Not suppose to fail due to the heat and stresses of speed. Mis-match of tires is like wearing a leather loafer on one foot and a hiking shoe on the other foot. The geometry is just wrong. Sure they both fit, but may have issues when you push them. Straight line speed stability is screaming at you to set the bike up correctly. Tuff to diagnose any issue until you have a baseline and the bike setup correctly. Of course there could be something else worn, damaged or lose with the bike and the tire issue exacerbates the issue into your symptoms. Sure sounds like this garbage began with your change to the Shinko/Michelin combo. That's where I would be looking. Either put a Shinko on the front or a Michelin on the rear. Only then will you know if the stability returns. No need to alter sizes if you were happy with the handling of the 120/70 190/50-17 setup from before. Only if you are looking to change turn in or ride height would I change sizes. Go back to the base settings, I say....