I used to mix the developer and at some point in time refresh it with a lot of fresh developer which changed the working solution and the next print looked very different.

At the moment I run with what I call "replenished old brown" as that is what my developer looks like - old brown. And I keep it from session to session. I mixed a liter of developer (two months ago) and I add 3-4ml fresh stock per 8x10" print and go on as long as I can and that is quite a few 8x10 prints by now. It took a few prints to stabilize but now it works very consistently. However, it is a bit early to call it a success.