I had the same problem with a "used" 95 SC400. Worse in the cold weather, and if other accessories were on (rear defroster, wipers). Two dealers took a total of two weeks to find the problem. The second one left the car run for the better part of a day, intermittently driving it around. They were unable to recreate the problem, but noticed that the + wiring harness from the battery was too hot to touch. That (and the battery) were replaced and the problem went away.
Good luck.
David