C70 electricity drop out

Hello NG in spring 2002 I bought a new Volvo C70 which runs perfect but got a heavy problem with electricity; especialy when driving in cold weather, electicity drops - no light, no nothing - although battery is charged. My garage assumed it is the ignition switch (not the v-belt!)and after a change it worked quite well until now. Has anybody got similar experiances? And maybe he knows the explanation for that. Thanks in advance for a helpful hint Joachim