I think it's safe to assume that when the cockpit/plane begins to shake/vibrate, you are flying too fast for present weight/conditions. I recently started flying the SU-25A and noticed that the cockpit will shake at higher airspeed. Once I throttle back/slow down, the shake goes away....so I assume that is from overspeed conditions. I am used to flying high speed/low drag aircraft, so it will take a little getting used to.
Next time when it starts to shake, slow down and once the shaking stops, note your airspeed and keep it around that number.