Cheverolet Caprice

Chances are something in the ignition switch is damp from sitting in the snow and then warming up (condensation). If the problem continues, in the dark, start the engine, open the hood and watch for arcing from plug wires, coil, distributor. If none, mist water on the components to see if that changes it. If it only happens under a load, have a helper watch for arcing while you put the vehicle in gear with your foot on the brake and the parking brake on. WHile your foot is firmly on the brake, give it gas to cause it to miss and have a helper (from the side of the vehicle) watch for arcing.

Let me know what you find.
Joe