You might try a shorter stem. Be easie rto keep your position at teh back of the saddle. Could be you are reaching too far whcih pulls you off teh seat. I had that problem, switched stems. If your stem is correct, the handlbars should cover the front hub axle as you are resting on the hoods. If the hb's are in front of the hub from your view, too far of a reach, need shorter stem.
You might also switch to a seatpost with less set back, a straight up post with clamp at top instead of curved back (just incase you don't know what setback is, some don't
) This willalso help you stay towards the back of the saddle eliminating some of the reach.
