Hi,
I'm a late beginner as well, started about 9 months ago. I used to have that problem, but find that I don't get it as much now.
I've noticed that factors that affect my consistency can be classed into two groups, basically environmental factors (external), and practice method (internal).
Find that a change of environment, possibly due to the following makes me play differently.
1. Using a different piano
2. People listening,
3. Background noises
I suppose that the only way to work with these problems is to get used to these changes. I used to get fear pangs, when my teacher asked me to play for her. Anxiety does funny things -- sometimes I do better than I normally practice, and sometimes everything falls apart. Now it's much better, I don't feel anxious playing for her.
The second big thing I've noticed is how I practice. I used to just bang away at the notes and lately have found that I sometimes contorted my arm in an odd way but was not aware it. That is where, a lot of mistakes creep in and where on some days I miss most of the notes.
I think that this is because I did not know or understand the motions (or technique) well enough for playing the piece. I was choosing a motion that is awkward and unnatural for myself. Moreover, I did not realized that I was doing this until after reading a number of posts in this forum carefully. Your body tends to make more mistakes if you force it to do an awkward movement. I find that consciously analyzing and experimenting with motions until you find one that you are happy with for the speed you are aiming at helps. After that, I practice the motion (just small sections) with the aim of producing a beautiful sound (playing in slow motion). This has changed the quality of my practice sessions dramatically. I do this repeatedly making sure that the back, chest, shoulder, arm, wrist and touch feels right -- fluid and graceful with minimal tension, at the same time bringing the section up to speed, setting the phrasing and trying out the dynamics.
I also find that warming up with some slow motion play, for a difficult section of the piece that I am are learning, puts me in the right frame of mind to practice. I basically start listening more carefully and try to consciously be more aware of the feel and the touch. At the same time, it reminds me of how it feels like to play that section, so that when I do practice it up to speed later, it comes more easily.
After trying all that, the problem with my inconsistencies just goes away! I think this is because my body knows what to do and is comfortable doing it!
Hope this helps!
al.
P.S. I have one thing to add, about being an adult beginner, that I've found useful. You can actually (usually) afford to buy a good piano -- one that you like. I've recently bought a reasonably good piano, and i think it has made me much more sensitive to the actions I'm using and the sound I'm making. It is difficult to connect action to sound on my previous, cheap, clunk of a piano. If you know what you are doing, playing on a good instrument does make a huge difference!