Originally Posted by
FIVE ONE SIX
your saddle's too far back, move it forward...
the same exact thing happened to me, and moving it forward helped...
keep in mind that may require you to switch to a longer stem. if you move your saddle forward 10mm, you need to increase your reach 10mm too, and they only way you can do that is with a longer stem...
However the OP may be moving forward while pedalling because the saddle-to-bar reach was too long to start with. I experienced a similar effect with a frame which was too large for me.