You're probably experiencing a momentary dead short somewhere in a wire harness that causes the alternator to go to full current output to meet the demand, and as the short corrects itself, the current surge has to go somewhere. It picks the lowest-resistance path available, and that's probably the heater motor if it happens to be running at the time. The sudden loss of voltage as the short occurs is what's killing the radio memory. Since the speed of electrical current is theoretically the speed of light, it doesn't take long for a surge to travel all over the vehicle at 186,000 miles per second.

Start your diagnosis by doing an inch-by-inch physical inspection of the wiring harnesses, looking for a worn spot in the insulation that's contacting the frame or body intermittently with one of the big, heavy-current conductors like a battery cable, alternator output wire, main voltage supply to the ignition switch or fuse block, etc.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
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