engine dying due to electric fan

hello all, hoping to get some help here and couldn't find this in any search. have a 340 engine, electric fan (seems to be hooked up properly, but I'm not good with electrical). engine runs perfect, until the fan kicks in, then it seems to be robbed, and dies. the only thing that makes me wonder is, that it's hooked into the ignition switch (which seems to be proper?). any help is much appreciated. thanks in advance. believe it's a vintage air thermostat with wiring kit. again, thanks.