I know that sounds stupid, but it sounds like you have too much grip. I imagine that the old servo was straighting out under power and so the car wasn't turning as sharp. But the new, more powerful servo, is holding it's line under power, the cars gripping too much and the only way to release that power/g-force is to flip the car.
Have a look at this for some tips:- (i didn't write this by the way, i got it from here http://www.rccartips.com/index.html)
Shock Oil
Use heavier oil in hot weather and lighter oil in warm weather. If
your rc touring car is understeering, try lighter oil at the front.
If car is rolling excessively, try using heavier oil at the rear.
Coil Springs
If understeering, use softer springs at the front. If oversteering,
use harder springs at the front.
Shock Position
For high speed tracks, place shock absorbers to a more horizontal
position. For technical tracks, place shock absorber in a more
vertical or upright position.
Front Stabilizer or Anti Roll Bar
This will reduce the side grip of the front end. It will reduce
chassis roll and may result in understeer.
Rear Stabilizer or Anti Roll Bar
This will reduce the side grip of the rear end and reduce chassis
role.
Increasing front track-width will result in more understeer, while
decreasing it will result in less understeer and faster steering
response.
In general, more suspension travel (less downstop) makes the
car more responsive but less stable. It is also better on a bumpy
track.
Decreasing the ride-height (lowering the car) gives you more
grip. However, because of changes in suspension geometry and
decreasing ground clearance, there are also negative consequences
to doing this.
In general, more negative camber means increased grip.
Nevertheless, it should always be adjusted such that the front
tires wear flat, while the rear tires should wear slightly conical
to the inside.
Toe-in is used to stabilize the car. In the case of oversteer (the
rear end losing traction before the front), adding extra rear toe-
in may help, but this makes on-power cornering a little more
difficult. If the car is too stable and pushing (understeering),
removing some front toe-in will remedy this.
Anti-roll bars are used to adjust the car's side- raction. In general,
increasing the hardness of an anti-roll bar on one particular axle
decreases the side-traction of that axle and increases the side-
traction of the other axle. For example, if you make the front
anti-roll bar harder, you decrease the side-traction of the front
and increase the side-traction of the rear. This will result in less
steering (more understeer).
Changing the front anti-roll bar has more effect on turning-in
(entering a turn, decelerating, off-power).
Changing the rear anti-roll bar has more effect on powering-out
(exiting a turn, accelerating, on-power).