Depending on the hill gradient and if it's safe, you could try going downhill on the commute using just engine-braking (use a low gear) rather than riding the brakes. This could help determine if the problem is brake-related or not.
Unfortunately it takes long enough to reproduce and is unreliable enough that I'm not willing to do enough trials relying on just engine-braking that I would be able to draw any firm conclusions. I have tried relying on gearing and I suspect it has something to do with the brakes but I don't know.
The last stretch where it goes down to 40 and then 30 is steep enough that engine braking without any brake pressure at all is not going to happen safely. And there are lights too, I almost always (but not always) hit at least a couple of them coming into town, so stopping is kind of mandatory there (and unpredictable).
I don't have a habit of riding the brakes though, I'm either gently depressing them or my foot is off. But without gearing down there's going to be a significant amount of intermittent braking; just not continuous.
