Aircraft unstable

When flying the PMDG T7, in flight it appears to becomes unstable and begins shaking/rocking/bouncing at high altitudes. Most recent example is followed steps in tutorial 1 exactly. Upon departure and reaching about FL 280 and speed of 260 the aircraft begins the shaking/rocking/bouncing. The shaking/rocking/bouncing behaves the same as when the aircraft is stalling. Upon reaching FL380 I must reduce speed manually to 235 or the aircraft will not stop shaking/rocking/bouncing.

I have attempted to uninstall and reinstall the software but it appears to be in the registry and therefore whatever is causing the issues is still embedded in the PC.  I am curious if this is happening to others or if this is an isolated event? 

Any support in attempting to ressolve this would be greatly appreciated.