Inconsistency Syndrome
You are inconsistent You hit one shot then blow the next or the one after that. You lose many more points to your unforced errors than to your opponent's winners. There is no shot you own that you feel you can rely on when the chips are down. Some days you wonder why you even bother to play this awful game.
I'm sorry, am I presuming too much? I don't think so. The preceding could easily describe most playing pros on a less than stellar day. Errors are an essential and unavoidable part of tennis. Trying to prevent all errors is pointless and dangerous to your game. Both success and enjoyment in tennis are dependent on minimizing errors. No matter how hard you hit, how fast you are, how beautiful your strokes - you still have to be able to hit two balls in a row over the net and into the court to win and stay sane.
First, and perhaps most important, you must abandon the fantasy of absolute control and embrace the reality of percentages.
Since unforced errors are universal, calling inconsistency a "syndrome" is like calling old age a syndrome. Just like old age, you cannot avoid inconsistency. You can manage it, but only if you embrace it first. The key is to stop fighting it. Understand that unforced errors are inevitable and the goal should be to mitigate them. The difference between avoidance and mitigation is subtle but important. Avoidance leads to over-cautious and defensive play, poor shot mechanics (see Rubber Elbow Syndrome) and a whole peck of psychopathology. Mitigation leads to victory.
First, and perhaps most important, you must abandon the fantasy of absolute control and embrace the reality of percentages.
Percentage Play
There are lots of uncontrollable parameters in tennis: The speed and spin of the incoming ball and the height of its bounce, the existence of a "sweet spot" and "dead zones" on the racket face. These and many other factors conspire to inject an exciting but frustrating unpredictability to a game that would be pretty boring without it. No one is meant to play error-free tennis. Knowing this, the smart or instinctive players embrace the chaos of the game and turn it to their advantage. They know how to push the limits of power, spin, placement, and depth without sacrificing any consistency. They always aim for the same set of targets, always hit a given shot the same way - not too hard, not too soft. They always hit with spin and never invent strokes on the fly. They never hit for the lines and never over-hit, but also never "play it safe." What would be the point? Every shot they hit is as safe as they can make it because every point counts.
So the best shot in the world is no good if it doesn't stay in play. On the other hand, it is also true - and sometimes this truth is tragically ignored - that a soft, short, flat, crappy shot is NOT more consistent than a hard, full stroke with the right kind of spin on it. Power is the source of pace AND control. There can be no control without some power and some spin. Go for too much power, control or spin and the ball will dive into the net or fly off into space. There are natural limits on how much of each of these you can generate efficiently and safely. What we need to do to stay consistent is to create and maintain a balance between pace, spin, control and placement.
Brewing Consistency
So step one in managing your percentages is to learn to combine just the right amount of power, control, placement and spin into a stroke that is easy for you to hit and highly likely to stay in play but is still effective. How does one do this? As humans we deal best with absolutes; more is better, less is more, etc. We don't love "a pinch of this and a dab of that," so how do we boil this down to "just enough power, control, and spin to achieve X"? The first thing to do is define 'X' very precisely. If you know exactly what you are trying to accomplish each time you strike the ball, then you can build, rebuild, and drill your shots with 'X' as your goal. Fail to achieve 'X,' and you need to adjust. Make 'X' consistently, and you are on the path to your A-game.
The 'X' Factor
'X' needs to be a simple, objective measure of the effectiveness of a stroke. Now, I might say that a 100% consistent, 100 mph forehand with 3000 rpm of topspin that lands on the backhand corner of the court is a good ball. Why can't I just do that? Well, there is no universe in which that is achievable (thank God!). Many B-players hit for the lines when they are trying to be aggressive, hit the ball 100 mph, albeit into the back fence, and lay on the topspin like it is a miracle cure for inconsistency. I have done all of those things at times. Interestingly, two of those parameters, taken by themselves, are achievable; pace and spin. The parts that do not work and will never work are placement and consistency. As noted above, 100% consistency is not possible in tennis. Less than 50% is not acceptable - you will always lose. So the sweet spot for consistency is somewhere between 50 and 100%. A very effective stroke, i.e. a 'winner', need be only 51 % consistent to win virtually every time. The less effective a shot, the more consistent it has to be. Thus many A-players hit the ball hard; increasing pace improves effectiveness more than it decreases consistency. On average 70-80% consistency is considered good for all strokes and the second serve, 60% for the first serve. If pace and spin don't radically effect percentages, how do you fine tune your consistency to get it into the desired range? The answer is placement. Not where the ball lands, but where you intend it to land.