Mid-range flutter can come from many different technique problems. The two problems that I see most are:
1) Ratio of snap (wrist speed) to arm speed being messed up. A lot of people try to throw the disc too hard with their arm, and don't snap the disc off clean at the end of the throw, causing flutter. This happens when someone is trying to "force" power on their throw. Wrist speed has to be greater than arm speed. If the disc is not spinning, it will not want to fly. At that point, the disc turns into a projectile, rather than a flying object with wing, etc.
Possible Solution: Relax your wrist (this may seem counter-intuitive, but it's definitely not). Slow down your arm speed. Keep the disc close to your body. Doing these three things will put much more torque on the disc, so after a few attempts you might be flipping the disc over. That is easily resolved with adjusting the angle of release. But the original issue was "flutter". Once you get the flutter out, then start messing around with release angles until your Buzzz, Roc, or whatever flies like you want it to.
2) Grip. This is a highly debated issue, mostly because there are many different ways to get the same result. If one grip works for player "A", then he/she will often try to convince player's "B" through "Z" that it's the "correct" grip. Really, any grip that attains the results you need is the right grip, and since our hands are different sizes, and we come from different frisbee backgrounds, it doesn't matter too much. There are a few semi-universal truths, however. For one, the "Power Grip" will usually result in the most power, but many people have serious accuracy issues with this grip. On the other side we have the "Fan Grip", which most people have the most accuracy with, but you will lose a significant amount of power because you can't apply as much torque. Then we have the "Control Grip", which is kind of a middle ground between "Fan" and "Power".
Some times, flutter issues with mid-range discs spawn from grip issues. A lot of people assume that they should use the same grip for mid-range shots as they do for drives. Truth is that some people just can't Power Grip a mid-range effectively. For any kind of throw, you have to be able to comfortably stabilize the disc in your hand when you throw, otherwise errors can occur upon release. Often causing flutter (or severe worm-burning, or 90 degree shanks, etc).
Finally, I'd like to dispel the myth that anyone can overpower a KC Roc. It's virtually impossible as far as I know. As long as you do everything correctly, a KC Roc should be able to handle just about any amount of power you can muster up.