I just received a recall notice the other day. Haven’t had the problem, but the notice literally said that there’s an issue with the vacuum pump where over time, it weakens so that at lower speeds, it’s less effective. Apparently there’s some hydraulic assist or something that’s only programmed to work sometimes. The recall involves the dealer reprogramming it so the assist picks up the slack. I’m sure I’m not explaining it perfectly, but after getting the notice and seeing this, I bet it’s your problem. I’ve never had the issue myself.