Now you're getting a little closer to the problem. Sometimes you play very well, and sometimes you don't. In other words, there seems to be a lack of consistency. Time to take a look at your routines. Do you approach every shot the same way every time? Do you set up the same way for every shot. Is your pre-shot routine consistent? How about your shooting checklist?
Generally speaking, when your game fails you, it's because you have changed something in your routine. When you practice, what do you practice? Are your fundamentals so ingrained that you don't need to think about them when you play?
If you can answer these questions, you are well on your way to bringing your average playing level up a notch or two. I have worked with some very good players who, when we got them on video, were able to spot small little cracks in their fundamentals that were holding them back. A little twist of the wrist, slight misalignment, or some unecessary arm movement that was putting a hitch in the stroke. It's rarely the big things that trip you up, it's more often those little things that you may not even notice.
Try getting someone who know what to look for to watch you shoot. Or set up a video camera and see what shows up.
Steve