Prevent cocoa from settling
You are fighting a losing battle here. Cocoa powder does not dissolve in water, and it will simply sediment over time. If you are unwilling to do something about viscosity (and I understand that - gums do change flavor), you can only slow the process by using methods which are increasingly difficult to apply at home (basically, creating smaller cocoa powder pieces). 

If you are not already doing so, boiling the sauce first will help somewhat. This is the same line of thinking as adding some starch (cornstarch or tapioca were mentioned, doesn't matter which). Cocoa powder itself is mostly starch, so once it is cooked, it will be more puddinglike and will bind water better. 

My preferred solution though would be to abandon the recipe and start working with chocolate instead. Chocolate is a sol, consisting of solid particles suspended in fat. Much like an emulsion, only both phases are solid. When you mix it with the fatty emulsion of milk, you achieve a much smoother mixture than the cocoa powder + water combination. It won't guarantee perfect dispersion, but should be better. Unlike the previous suggestion, do not boil or you will lose the advantages of the existing sol/emulsion.