I had these issue with my tricopter and I could improve its stability by changing the PID values for pitch and roll.
My tricopter setup is the same as the shop provide, all purchased in rcexplorer.se except for the receiver of course.
I started with the PID values that David posted in the guide, then I raised P values a lot and its behaviour improved.
Here you have my PID setup:
And some videos of the result: