1998 Chevy Blazer voltmeter pulses (lights dim, fan slows)

Beginning a couple winters ago, the voltage to my '98 Blazer's lights, fan, etc begun to pulse - for a few seconds it is fine and then for a few seconds the voltage drops, the lights dim, the fan speed slows, etc. This only occurs during cold weather and essentially renders the vehicle undrivable during the winter months. During the spring, summer and fall I've experienced no problems. The local dealer has installed three new alternators, declaring the problem solved each time until the vehicle leaves the warm shop and then the problem reoccurs. They've also checked for poor ground and replaced a battery and cable but to no avail. What is happening differently in cold vs warmer temperatures that would cause this pulsing voltage? I would really like to have this vehicle available for winter use and am hoping you might have a solution.