So my saddle rails position was set up for knee over pedal in a shop. It all feels good when I'm properly back on the saddle, the width is good and pedalling feels smooth.

During the ride I keep finding myself slipping forward, which leads to the obvious resulting discomfort. I'm fine when I slide myself back onto the flats.

I think the saddle fore and aft is right, it's level and the height feels right.

My suspicion is that my stem is too long and that as I reach to get on the hoods, I wind up creeping forward on the saddle as I pedal.

Does this sound...sound?

Trying to sort out what might be causing me to slip forward on the saddle over time.

You can tell from the pic that the seat is a bit forward on the rails and I don't think due to pedal position I should move it any farther. If I measured the stem right, I measured center of the steerer to center of the bars at 100mm, I think I can drop to an 80 or so and that might "push" me back onto the saddle properly.

I have to say, this is the first bike I've ever owned where a bit too big was the issue.