Hi all and thanks for the good replies on my question.
I tried some different ways of solving this problem and
the solution that works best for me is the following:

I bought a slightly hotter plug, this advances the ignition
point slightly, however by increasing the needle mixture
the engine gets more fuel. More fuel cools the engine which
delays the ignition point. These two factors takes out eachother
so the hotter glowplug makes my
engine run on a richer mixture at the same ignitionpoint.
And now it runs perfect even under very cold conditions.

Thanks all!