Measure voltage over the coil when the fan kicks in.
It's also mostlikely the added power-draw of the fan is taxing the alternator so that in turn will bog down the engine. When the rpms drops, so does the alt output and it's all downhill from there.
I've got a 360ci with a modern Nippo alternator on an engine teststand. When I flick on the relayed fan's switch it also bogs the engine down noticably in the beginning.
A 'slow-start' PWM fan-controller would help greatly as well as it doesn't shock the electric system so much.
