might be helpful to know exactly what conditions you are shooting in... cold is how cold? is the weather inclement, or dry? the cold must be causing the lens mechanism to shrink enough to affect the AF mechanism's calculations -- which are probably tuned within strict tolerances to achieve acceptable focus. i can't say i've ever had problems like you describe, even though i've been out shooting in very low temps. still, your cold temperatures could be far, far more frigid than what i've experienced.