Does anyone experience these ups and downs?
Yes, everyone does.
And if so, is there a way I can help my practice to be more consistent and stable?
Yes, keep practicing. Don't judge the success of your practice by what you experience. Instead, ask yourself, "Did I do my best given the circumstances?" Let it humble you and be happy you did your best. Let the practice itself be unconditional, but don't expect results to be unconditional. . . yet.
I'm not sure at those times whether to increase practice, keep it the same or decrease it...I just don't know what to do.
Try it both ways. There's no rule about it. However you approach it, you will learn about yourself, that is what is important.
Also, I should add - depending on what kind of difficulties you have, you can tailor your practice to your needs on any given day. For example, sometimes you may need more shamata, or more reflecting on the 4 thoughts, or making more aspiration prayers, etc.