I have a problem when I pull back. The blade "buckles" and flaps back and forth.
I take this to mean that the far end of the saw (on the opposite side of the cut) flaps back and forth as you saw?
It's really all about the application of the force. If you don't apply force in line with the blade, you're going to get odd vibration and movement, and your cuts won't be straight.
Also, your saw could be binding in the kerf. This is either caused by the kerf closing in on itself due to the wood warping or just friction between the blade and the wood. In the latter case, your saw could have an insufficient amount of set or just needs a quick swipe of lubrication along its length. This friction will set up resonance along the length of the blade when you pull back, hence the wiggling on the far end.
As @keshlam says, let the weight of the saw do the work and don't force it down. That will definitely buckle the saw when pushing it through the cut and could contribute to the friction issue I highlighted above.
And, as always, make sure your saw is properly sharpened and correctly set for best results. I can't tell you how many times I've had a saw behave badly because I failed to do these two things.