I had the same problem as you. Lots of people here will say that you need more top spin -- but its more than that. It's also trajectory off the strings, swing speed, swing path, and timing. It's the whole enchilada, so to speak.
There are a few factors here at work:
1. Your racquet is generating a lot of power
2. YOU are generating a lot of power
3. Your strings are generating a lot of power
what this means is, you add all of those things together and you'll find that you cannot compensate for the tons of power with merely adding more top spin. You've got to do more than that.
I agree with SundayPunch, you might want to switch to poly. What you have to do is find ways of counterbalancing all the power that's in your arsenal. This can be done by low powered strings. Or, you can string regular syn gut very tight (60 lbs plus).
Secondly, you need to look at your swing speed -- until you master everything else, you've got to slow down your racquet head speed. You're generating too much pace.
Thirdly, you've got to attack the ball sooner. You should hit the ball when its in front of you, not when it eventually gets even with your body. The sooner you hit the ball (on the rise), the lower the trajectory the ball will take when it leaves your strings.
Lastly, as everyone else says: top spin. You should modify your swing path so you're going more low to high, as opposed to hitting through the ball. Really brush up on the ball so that you are applying enough spin to compensate for all the power you're generating.
For my part, I could have done all that I just suggested to you, when I was using powerful tweeners. But I took the easy path out, I sold them all and bought the lowest powered racquet I could find with the lowest powered strings so that I didn't have to modify my swing speed or top spin. Now 90% of my shots stay in, which is great for me.