A slightly tight level 10 setup can cause this. The colder temperatures can cause the carriers to tighten the oring just a tiny bit causing a slower push when first firing the bolt. Using a lighter bolt spring will reduce this effect. That way you don't have to worry about changing carriers for different temperatures. Its easier to change bolt springs for cold temperatures than carriers. You might be able to use the next larger carrier size (check for leaking), but remember to put the original one back in for regular temperature play.
Except for the Automag in front, its usually the man behind the equipment that counts.