I am going to just start going through the plethora of chemicals at home to see if I can find the magic formula. Goo Gone works well, but it also usually removes more than I hoped. :) Alcohol might work. Maybe a heavy duty degreaser? Mix them all together and hold your breath....literally.

Let me know what you figure out. I have an Italian shotgun with a "rubberized" synthetic stock that did the same thing. Anywhere the stock was touched essentially melted, the rest is as good as new. Since the parts are no longer available the thing is worthless unless I can repair it.