I have the same problem shooting that "old fashioned" overarm method now its warmer. I have tried several times to shoot off the knee -- the rifle wobbles all over the show, my legs are tense and I cant breathe! Shooting overarm I am relaxed and steady ( apart from my pulse bumping the cross hairs up and down in the kill area - you get used to that!).
Whats the secret to off the knee? Several club members have shown me what they do - but I just cant get on with it.